What is dark matter?
It’ll take a lot of modern physics to understand the present notion of dark matter. I will thus refrain from answering it in technical detail.
This is what you should know:
Dark Matter is not matter at all. Nor is it something you can touch, see, feel or sense. We Astrophysicists understand very little about this mystical quantity.
What we do know is this. If you account for the gravity exerted by every single object in the Milky Way Galaxy (be it planets, stars, or black holes), the sum is just a fraction needed to keep together in the well known spiral shape. We cannot account for more than 90% of the gravity!
So where did all this extra gravity come from? And why can we not see it?
We don’t know. Yet.
When light passes through empty space, sometimes it bends around something invisible. This phenomenon is called gravitational lensing and stands as proof for the existence of dark matter.
We presently think of dark matter as something that does NOT interact with normal matter (except gravitationally) and with itself.