A galaxy is a system of stars, interstellar gas, dust, and dark matter that rotates in a curve like pattern called a velocity curve. Physists have tried to explain a process called “dark matter self-interaction” by explaining that a graph of rotation speeds at different distances from the center can be easily explained if dark matter particles are assumed to strongly collide with one another in the inner halo that is the place close to the galaxy's center.
Two different galaxies that appear at first instance to be very similar and very much identical can have inner regions rotating at very different speed. It is further explained that every galaxy sits within a dark matter halo that forms the gravitational platform holding them altogether. This gravitational force allows them to rotate in an identical pattern without losing any of the particles, the distribution of dark matter in that halo depends on the motion of stars and gas particles in the galaxy.
In the Cold Dark Matter theory (CDM), dark matter particles are presumed to be collision-less, aside from gravitational force. The dark matter collisions take place in the deep inner halo, where the luminous galaxy is established. When the particles collide, they exchange energy and become thermal neutrons. Present observation paved the way for future observational, experimental, numerical, and theoretical work.