The problematic rotation curves of galaxies is the finding that led to the hypothesis of dark matter in the first place.
That's fine, but scientists assert that DM comprises 70% (or more) of the universe, and that assertion seems to me to imply that in any given "clump" of matter, 70% of it will be DM and 30% of it will be "lit" matter.
You're forgetting that DM doesn't interact with matter electromagnetically. It only interacts gravitationally. And gravitational interactions such as this are extremely difficult to detect, except by looking at large, massive objects such as galaxies.
It kind of sounds like you are arguing from incredulity: "it doesn't seem right, so I don't believe it". But science doesn't operate on gut feelings. If you were to actually read up on the challenges astrophysicists are grappling with that led them to the DM theory, you might begin to understand why it is the leading theory.
Red:
I'd say I'm more skeptical -- far more "What choo talkin' 'bout, Willis?" than "Oh, hell no!" -- than incredulous. My comprehension of DM isn't great enough for me to have so firm a stance as incredulousness. Thus I'm not so much arguing anything as I am sharing what I've learned/heard (which, frankly, is nothing beyond very basic physics) and what be my understanding of it, and, in turn, asking tacitly what's amiss with how I'm applying what I've heard/learned.
I realize I may not have heard every "plain language" explanation or theory of what DM is, where it is, how much of it there is, etc. For example:
- NASA says, "Together, dark energy and dark matter make up 95% of the universe. That’s almost all of it!" (95%, 70%, half...the specific sum doesn't matter too much because, proportionally, it ranges from most "stuff" to the overwhelming preponderance of "stuff" to damn near everything.
- Einstein said e=mc^2 -- matter and energy are the same thing.
- Physicists (I say "physicists" on account of your having said so, and I'm trusting you wouldn't misrepresent what they say) say the universe is, by and large, "flat," which I take to mean "stuff" is evenly distributed in the universe.
- Physicists (I say "physicists" on account of your having said so, and I'm trusting you wouldn't misrepresent what they say) say that DM interacts gravitationally, but not electromagnetically, which I think means it behaves/manifests sort of the way rocks on Earth do.
- Considering the above bullets, I think to myself "Okay, fine. I'm willing to believe those statements, but let me see if I can reconcile them." To do so, I think about them as follows:
- If DM comprises most to substantially all the mass in the universe, it ought to do so evenly everywhere because the universe if "flat." That,, in my mind, means it ought to literally be between my house and the neighbor's, and between the Earth and the Moon, and so on. And since DM interacts gravitationally, the "bunch" of it that's between my house and the neighbor's should pretty much stay put -- perhaps not literally between our houses, but at least around the planet, as does the atmosphere -- because the Earth is a much bigger rock.
So despite DM's purportedly being ~70% or more of the mass that's, well, everywhere (right here at my desk, across the street, etc. being part of the universe, thus part of everywhere), physicists can't find any of it. If the universe be "lumpy," it'd make sense that DM is "over there" but not "right here in front of my face," so to speak. Lumpiness would account, in my mind, for DM being clustered in some parts of the universe and not in others, and I'd understand why one might not find some "right here" or in some other specific place where one might look for it.
My notion of "lumpy" conceives the universe as room having marbles and balls that move around and in certain places there are tables and in others there are "free floating" curtains, and the chairs and curtains are DM and the balls and marbles are solar systems. A moving ball or marble won't always encounter a chair or a curtain, but if the two combined comprise 70%+ of what's in the room, one's going to encounter one or the other of them. If, however, DM is like the air in the room, one's definitely moving through it, and it's effect will be noticeable by the fact that it slows the motion of the marbles/balls.
That concept brings me to the another troubling part of the explanations/descriptions I've heard about DM....gravitational, but not electromagnetic....
- Since one can consider mass as the charge of a gravitational interaction, it stands to reason that where there are electric charges, electromagnetism either is happening or can happen. Yet I'm bid to accept that DM has mass and has no charge. Surely you can see how, at least to me, the notion that DM interacts gravitationally and not electromagnetically seems to contradict the proposition that DM has mass?
Hopefully the above clarifies the nature of stance on DM. I'm not of a mind that it doesn't exist. I'm of the mind that what I've been told about it doesn't all fit together, and it since it doesn't, the "story" of DM's nature strikes me much as does the "pretzels" into which ancient-to-early-Renaissance philosophers attempted to make geocentrism "work." Can I say what may be amiss in the DM theory? No. I can only say that if it's right, the explanations people proffer wouldn't strain credulity.
Now, it may be that DM theory does gel with what I know and that it's been inaptly depicted in the lay world. The depictions of DM certainly wouldn't be the only thing I've heard physicists say that didn't make sense to me. To wit, long hearing "nothing comes out of black holes" and seeing depictions of gamma rays and other such emerging from black holes....
...I thought those two things don't reconcile. So, what was wrong?
I emailed a physicist to engage in a conversation about the incongruity between the remarks.
He responded with a sensible explanation. People talk about the beams such as the one pictured above coming from black holes when in fact they're coming from "near but just outside" the black hole, far enough that we can observe gamma rays shooting out. Specifically that the X-rays and gamma-rays "from" black holes are produced OUTSIDE the event horizon, by the superheated gas racing around the hole, the accretion disk. The rays are distant and powerful (energetic) enough that they can escape the hole's gravity. (He mentioned Schwarzfield radii, which, frankly, was of no use to me, but his plain language explanation most certainly did and it made sense.)
So while I'm no physicist, not by any means, I can tell when what folks say doesn't line up. What I cannot tell (as go physics) is whether the exposition I've been given is "sloppy" or whether it's not sloppy and just doesn't make sense.
. And that 80% of those living in China are not actually Chinese?
