We’ve never seen Dark Matter. But we know that there isn’t enough matter in galaxies to hold them together, so there must be something adding more gravity; that’s what we call dark matter. The problem is that the only effect it seems to have is gravitational. Light goes right through it. For all we know, matter can go right through it.
But it’s may not be quite so mysterious after all. Astronomers may have observed dark matter having an effect beyond just gravity. Researchers have been studying 4 galaxies colliding about 1.3 billion light years away. It seems that a dark matter clump is lagging behind its galaxy by about 5,000 light years. This is predicted if dark matter interacts with other forces. It seems the friction from the collision would slow the dark matter down.
Astronomers can see the dark matter clump because they are viewing the clump through a gravitational lens from galaxies between us and the galaxies being studied.