Bigger galaxies. And stars.
If any galaxies actually collided on August 16, 2011, we won't know about it for some unknown number of millions of years, as the light of the collision reaches us. If we have just today observed that two distant galaxies appear to be colliding, then they collided some number of millions of years AGO, and the light of the event has just reached us.
They collided with another galaxy
The possessive form for the plural noun galaxies is galaxies'.Example: Their galaxies' paths do not coordinate.
Collider and collision are the noun forms for the verb to collide; collided is the past tense of the verb.
An irregular galaxy is a galaxy that doesn't have a specific shape like a pinwheel or an elliptical galaxy.
Planetesimals.
Dust and gas particles in a protoplanetary disk collided and stuck together to form planetesimals, which then collided and accreted to form planets. The process of gravitational attraction and collisions led to the formation of larger bodies within the disk, eventually forming planets.
Collision is a noun not a verb. The verb form is collide, and the past tense is collided.
Not if, not even when: galactic collisions are very,very common. Rarely the galaxies will just pass through the others; more often they will result in some combination of merging and then tossing out the matter that lies at the gravitational perifery.
Galaxies
That's called a supercluster.
The moon formed when a celestial body collided with Earth, causing debris to be ejected into space. This debris eventually came together to form the moon.