Q: What role does Dark Matter play in the behavior of things inside the solar system?

Physicist: To a stunningly good approximation: zero.

The big difference between dark matter and ordinary matter is that dark matter is “aloof” and doesn’t interact with other stuff.  Instead, it cruises by like “ghost particles”.  Matter on the other hand smacks into itself and clumps together.  The big commonality is that both of them create and are affected by gravity.

If you have a big ball of matter both ordinary and dark matter will be pulled by its gravity.

If you have a big ball of matter (doesn’t matter what kind), then both ordinary and dark matter will be pulled by its gravity.  However, there’s no reason for the dark matter to ever fall out of orbit since there’s nothing around to stop its motion.  Normal matter tends to “get in its own way”.

In fact, if it weren’t for the gravitational influence of dark matter, we would have no reason to suspect its existence at all.  Because dark matter doesn’t clump it stays really spread out and forms one big, roughly spherical, cloud around the galaxy.  Matter has more of a “big-clump-or-nothing” deal going on.  If you start with a big cloud of ordinary matter, then eventually (it can take a while) you’ll have one or two huge chunks (stars, binary stars, that sort of thing) and the few crumbs that escape tend to end up clumping together themselves (planets, moons, comets, your mom, etc.).  If you feel like impressing people at your next science party, this is called “accretion“.

Any attempt to picture the Sun and nearby stars to scale look like nothing at all.

Any attempt to picture the Sun and nearby stars to scale look like nothing.  This is an attempt where every square is 10 times the size of the previous square (1000 times the volume).  Point is, when ordinary matter concentrates it really, really, really concentrates.

In the above picture the dark matter is spread out uniformly.  Overall there’s a lot more of it (about 10 times as much, give or take), but here in the solar system the balance is tipped overwhelmingly in favor of ordinary matter.  But more than that, since dark matter is spread evenly (and thinly) all around us, it doesn’t pull in any particular direction.  There’s about the same amount in every direction you point, so there’s very little net pull in any direction.  Until you start considering galactic scales at least.

Here on Earth we can point straight at a few big collections of matter.  The most important is straight down, and the

Ordinary matter clusters in big blobs, so when it pulls it tends to pull in one direction (right).  Dark matter does pull, but it pulls on every particle evenly in every direction, which is a lot like not pulling at all (left).

If you do consider things on a galactic scale (~100,000 lightyears), then there’s more dark matter in the direction of Sagittarius (in December this is overhead around midnight).  Technically, since we’re most of the way to the edge of the galactic disk, and the center of the galaxy is behind the stars in Sagittarius, most of the stuff in the Milky Way is more or less in that direction.  That imbalance makes the Sun and all the other nearby stars (“nearby” = “visible to the eye”) orbit the galaxy, but it also helps Earth and everything else around us do the same.  Astronauts in orbit appear weightless because their ship and their bodies are both orbiting the Earth.  They are both in “free-fall”.  Similarly, the Earth, the Sun, and even everything in our stellar neighborhood are all in free-fall around the galaxy.  So while the preponderance of dark matter in the galaxy does cause the solar system to slowly sweep out a seriously huge circle (the “galactic year” is about 250 million Earth years), it does not cause things in the solar system to move with respect to each other.

Hopefully, dark matter has more tricks than just gravity.  If it has no other way of interacting with stuff, then that makes it really difficult to study.  We can study things like stars, rocks, and puppies because they’re all “strongly interacting”.  Shine light on them?  Sure.  Poke them?  Why not.  But dark matter (whatever it is) is light-proof and poke-proof, and that’s deeply frustrating.

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21 Responses to Q: What role does Dark Matter play in the behavior of things inside the solar system?

  1. John says:

    Given that dark matter responds to gravity, shouldn’t it be attracted to the sun? Wouldn’t you expect a large amount of dark matter to be sitting at the core of the sun?

  2. Manny says:

    Finally an explanation so that everyone understands the difference between dark matter and matter.

    I do have another question: it is said that since a black hole is a singularity you cannot actually reach its center, so what happens with the matter the black hole is able to eat? Will that matter simply clump on the edges of the event horizon, inside the black hole or what? Thanks.

  3. ivo says:

    great article!

    p.s. shouldn’t be “role” instead of “roll”? am I missing a pun?

  4. Flavian Popa says:

    Very good topic. I wonder, had dark matter ever been stuffed in one tiny place, could have it generated black holes made up by dark matter accumulated in one tiny singularity? Just a wild thought…

  5. Mehrdad says:

    Interesting, how come even the tiniest amounts of dark matter (or neutrinos) don’t collapse into a blackhole? As they respond to gravity, but not the EM force to keep them apart?

  6. The Physicist The Physicist says:

    @ivo
    Thanks for catching that! I’ve been writing about dice too much.

  7. The Physicist The Physicist says:

    @Flavian
    Enough matter, dark or anti or whatever, in one place and you’ve got a black hole. However, there’s no good way to get dark matter into one place.

  8. kopernik2 says:

    Dark matter engulfs galaxies. Adds to the gravitational attraction between distant galaxies. Most galaxies are rushing to some point of convergence. When these do meet, sometimes hundreds or thousands, their trailing dark matter halos have so much momentum that they shoot past the group of galaxies. See the Bullet conglomeration or this – 1E 0657-56, X-ray NASA, CXC, CfA M.Markevitch et al. Optical NASA, STSc Magellan, U.Arizona, D.Clowe et al. Lensing NASA, STScI. ESO WFI. Magellan U.Arizona, D.Clowe et al.

  9. kopernik2 says:

    Re: Manny,
    As I understand it, when matter accretes onto a black hole (well below the event horizon) it forms strata (palimpsest layering). E.g., the additional mass of a star would make a splat, probably not cover all the black hole.
    Density of the BH would decrease towards its center. All of the BH is slowing ‘shrinking’ or becoming more dense.

  10. The Physicist The Physicist says:

    @kopernik2
    Sometimes galaxies do collide (our galaxy and Andromeda, or the bullet galaxy you mentioned), but there is definitely no “point of convergence”. Overall the galaxies in the universe are getting farther apart, with the rate approximately proportional to the distance between them.

  11. The Physicist The Physicist says:

    @John
    You’re right; dark matter is pulled by the Sun. However, with nothing to stop it, being near the Sun just means that it’s moving faster. You can think of it like a comet: slow far away, fast up close.
    Somewhat unintuitively, it turns out that this actually makes the dark matter density near stars lower. Isn’t that weird?

  12. maliha says:

    wow! this is really cool! how do you figure these things out?!

  13. Mind2Matter2Reality says:

    “Hopefully, dark matter has more tricks than just gravity”.
    Exactly! Gravity is a “trick”! It’s a magic trick, that defies the conservation of energy.

    All the theories on “gravity” (the observation that things tend to move towards each other) have only been descriptions of the “gravity” effect and not true explanations of what it is and why it should exist.

    Without understanding why things “gravitate” towards each other, today’s “science” is just in the business of magic and illusion, making up fictional energies, forces, particles, and the negatives of those.
    No different than Hollywood, just different “actors” and different sci-fi titles.

    For the lovers of ‘You Can’t See it, But Believe Me It’s There’ genre the latest and greatest sci-fi movie is “The Dark Matter”. It looks like the theaters are all sold out since there is no lack of “physicists” who believe in magic.

    I’m still waiting for the old classic “The Gray Matter”. There is so little of the gray stuff left these days that you need a microscope to find it.

    Form –> Function
    Geometry is what shapes the universe and the universe functions based on its shape.
    Very simple.
    K.I.S.S.

  14. Mariano Quiroga says:

    Are neutrinos part of this so called dark matter? Or could have been part of this “Dark matter” until they were detected? Could dark matter be formed by every kind of still unknown particles, including the recently known Higgs boson?

  15. Jordan Sussams says:

    Perhaps I am missing something fundamental here, I am relatively new to educating myself in physics, but I couldn’t help form the idea that if dark matter has enough gravitational influence to hold galaxies together yet ignores light, is it possible in theory that dark matter can move faster than light particles and if so then is it reasonable to assume that when matter is drawn into a black hole it is somehow converted or super compressed into dark matter and simply escapes back into the universe, I know ordinarily nothing escapes after breaching the event horizon but isn’t that just because nothing can move fast enough?. If I sound ridiculous then enjoy the good laugh but it’s bugging me.

  16. Jesse Jaramillo says:

    Has dark matter even been proved to exist? People here are talking about it like they are experts. I have heard of no confirmation that it even exists.

  17. Xerenarcy says:

    I have huge problems with the notions of dark energy and dark matter. For one, the person who came up with the concept of dark matter had another theory that has been laughed out of the physics community – tired light.

    at the scales required to observe distant galaxies and such, we rely almost entirely on the properties of light to carry information back to us about the distant objects. based on relativistic doppler shifts in frequency, we construct redshift values for distant objects to figure out how far away they are.

    however… this relation relies on the fact that light, when traveling through flat space, remains unchanged. therefore if we were to observe a relation between redshift and distance, assuming the properties of light, we would conclude that space is expanding in all directions.

    if that is the case, why doesn’t the universe just keep growing? gravitation yes. but observable matter is not enough to explain the strength of the ‘pull’ of ‘stuff’ in the universe that acts against this relentless expansion, based on what we can observe. throw in other discrepancies with gravitation and we start noticing an absence of matter relative to the gravitational effects.

    (i forget the name) one person suggested that there is some sort of weakly interacting (or not at all) type of matter that only is affected by gravity. be it neutrinos or something more exotic, not important for now aside showing why dark matter is needed. dark energy then becomes the opposing expansive force to gravity of matter and dark matter.

    this same person who thought of dark matter, came up with an alternate explanation for what we are observing – what if light can redshift over distance in flat space?

    sounds insane right? how can light change frequency, specifically lowering in frequency, over greater and greater distances…

    for the frequency of a photon to change, its energy must change, and in the absence of the lost energy being radiated away somehow it violates the conservation of energy. this (and some more technical arguments) is why the theory was thrown out, but its premise remains as likely as dark matter for explaining what we see when looking that far into space…

    consider that if light redshifts on its own, it would explain why distant objects are redshifted without the need for an expanding universe, dark energy or dark matter (in this context anyway, dark matter can be made obsolete with galaxys’ observed motions of stars explained by time dilation being greater towards the center of the galaxy, consistent with the belief that there is a supermassive black hole at the center of most galaxies including ours).

    the big question i have not seen taken seriously is what makes us assume that light cannot decay over time (distance)?

    something to think about at least… the conjugate variables involved with E = pc = hf are open to being subject to uncertainty relations and possibly even relativistic effects.

  18. Flaming Svin says:

    Hmmm,Good explaination

    If Darkmatter doesn’t interact with itself nor the other types of matter then it is likely that it is the ash version of visible matter.Visible Matter has electric and magnetic properties that makes it interact with stuffs while dark matter doesn’t have that properties.

  19. Stuart Shepherd says:

    2nd dumb article by “The Physicist” (although “PC” in the Physics field). “Dark matter” is a scam by the Physics “Industry” (education is an industry, isn’t it?) to obtain government funding for arcane and highly expensive “experiments” to prove it’s existence, when they wouldn’t describe anything else as scientific “fact” from mere suspicion as a possibility without much much more solid theoretical and/or experiemental evidence. Dark matter doesn’t exist. There is no such thing. The “gravitaional inflences” prompting the theorization (word?) of its’ existence are much more logically explained, for example, by the existence of a 2nd “universe” caused by a second singularity formed at the time of the Big Bang from the enormous counter-expansive forces. This “2nd universe,” exerting gravitational forces on ours, has simply not broken through to “visibility” in any way other than these influences. Why would you propose an entirely “new” and undefinable type of matter as the primary constituent of the universe when so much is already known and it doesn’t fit ANY of what actually is known an you have no evidence for it other than gravitational influences?! It’s almost like children playing a game and one decides he’s going to make the rules “because I said so.”!

  20. Stuart Shepherd says:

    But “The Physicist” is still the best and really a great man for providing all this brilliant teaching and discussion free of charge. I am NOT accusing him of “scamming,” but it does appear that he’s following the Physics status quo. I’m sure he’s confident enough in his own scientific positions to recognize counter arguments as just that and not personal attacks, and the use of words like “dumb” (whih I use frequently and probably shouldn’t) as anecdotal and not literal (because pretty much the OPPOSITE description would need to be used literally).

  21. The problem with dark matter is that it’s existence was “déduced” to explain the speed of stars on the farther part of a galaxie. The galaxie needed more matter than it had, to explain the observation.
    So they added matter that cannot be seen (matter we saw whasn’t present in sufficient quantity). This adding of matter is suppose to settle the problem.

    But I don’t think it really does, because those stars farther from the center of the galaxie have the same speed than the stars a lot closer to that center. So that the law of gravity which says that “gravity diminishes proportionaly to the square of the distance” doesn’t apply in a galaxie.

    As I see it, the problem is not solve at all.

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