1.   So for a sufficiently large number of matter particles, gravitational forces can dominate over all other forces.

2.   The force-carrying particle then collides with another matter particle and is absorbed.

3.   The recoil from this emission changes the velocity of the matter particle.

4.   The uncertainty principle also predicts that there will be similar virtual pairs of matter particles, such as electrons or quarks.

5.   This collision changes the velocity of the second particle, just as if there had been a force between the two matter particles.

6.   What happens is that a matter particle, such as an electron or a quark, emits a force-carrying particle.

7.   Loosely speaking, then, we live in a world of matter particles and energy particles.

8.   One possible solution is to assume that the dark matter particles are self-interacting, bouncing off each other occasionally like billiard balls through some other force.

9.   So astronomers think it must be the result of dark matter particles colliding.