Since they do not stick after collision hence collision is elastic. In elastic collision, both momentum and kinetic energy is conserved because in this type of collision, first body deforms but then quickly regains its former shape and transfers its kinetic energy to the second pluck.
So kinetic energy is conserved.
Galaxies are much further apart than stars. This is the reason why they are less likely to collide and the likelihood of galactic collision is much smaller than the likelihood of stellar collision. Example for galaxy collision is the collision of the Milky Way galaxy with Andromeda. It is estimated that the collision will be <span>in about 4.5 billion years. </span>
