Smaller dark halos fall into these large structures and form clusters of galaxies. Yet another close up only 40 megaparsecs on a side. We can see the violence of the collapse of density fluctuations and their relaxation into dark halos. A virtual spaceship flys through a single snapshot of this simulation representing a time about 5 billion years after the Big Bang. The dark halos are clearly apparent as blobs of particles and the filamentary structure which develops as a result of gravitational instability is also obvious in this ride.
Spiral galaxies are great collections of stars moving on nearly circular orbits with a disk.
Gravitational effects cause them to develop the spirals see. When two spiral galaxies interact, the two disks are dramatically altered by the mutual gravitational tidal fields of the two galaxies. However, the process of interaction takes orbital energy away from the two galaxies and causes them to merge and form a single, featureless galaxy with an ellipsoidal shape. It is widely believed that their origin probably is in the merger of two or more spiral galaxies. The animations portray the collision of two galaxies as viewed from three different perspectives and then repeated again but as viewed from a larger distance.