It has become apparent in the last decade that galaxies,
many galaxies, are seen colliding. The gravitational
pull of dark matter around galaxies and in galaxy clusters helps
to pull galaxies together and collide on relatively short
(cosmological) timescales of a billion years.
However, the large distance between stars in galaxies are large enough to
make the probability of stellar collision low.
Another widely recognizable gallactic collision product is the "The Cartwheel Galaxy". The Cartwheel is part of a
group of galaxies about 500 million light years away in the constellation Sculptor.
Colliding Galaxies Movies
and downloadable software