We think of the earth as gravitationally bound together, and it is, but, what gravity does is "pull". That's it, it "pulls you in a direction".

What stops you, are the other forces.

Normal matter just stops when it encounters other matter.

It can't pass through.

So, gravity pulls everything together, but the fact the stuff interacts means it clumps. It forms a big ball.

But dark matter doesn't stop when it hits the surface of the earth. Or even when it hits the middle, it just keeps going.

In fact, it just goes straight through the earth, as if the earth isn't even there.

The force of gravity isn't strong enough to hold fast moving particles that don't bang into things.

They just float away.