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Matter we know is there because of its gravitational effects but doesn't interact with light so we can't sense its presence directly. Beyond that there is only speculation, usually on particles that are massive (and thus have gravitational interactions) but don't interact electromagnetically.
As Jay inferred, we know more about what Dark Matter isn't than what it is. It can't be directly seen, but it does provide an expanation to what holds galaxies together. We see the effects of Dark Matter. The point there is that galaxies don't have enough matter to create enough gravitional attraction to hold galaxies together. As galaxies rotate like a merry-go-round, stars should be flung out. But that's not the case. Something seems to cause them to remain in the galaxies. Dark Matter is suspected to surround galaxies as a halo which keeps the vast majority of stars from flying out of the galaxy.
There have been searches for signs of Dark Matter inside our own galaxy, but so far haven't provided much promising results. That may suggest that Dark Matter resides on the outside of the galaxy. But we really don't know for sure. Dark Matter could be exotic particles or even a combination of different things.
Dark Matter is thought to compose about 24% of everything in the universe. Not only does it hold galaxies together, but it is likely to be involved with the formation of filaments that make up the cosmic web of the universe. Here are a couple of links and a vid that may help give some basic thoughts and ideas about Dark Matter.
I asked myself the same question once. Spent a good day or two casually researching. My findings??? I have no clue whatsoever, and apparently, neither does anyone else
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