>it is thought that the universe is just one of many that started, they all resided in a cosmic soup and due to particular circumstances the dot in the soup that is our universe started to expand in the big bang. the universe has been proved recently to be expanding forever (thats not to say that someone will prove otherwise in the future), with the result that the matter in the universe will spread out creating a less dense universe as time goes on. this will take trillions and trillions of years, and will be after the last black holes evaporate. i have never read anything that contemplated what would happen if two universes collided, but i imagine that this would bring eternity to a stand still in our universe if it did

The possibility of collisions would depend on whether different "universes" are part of the same three-dimensional space, or whether the three-dimensional (or ten-dimensional) structure we live in is only part of a higher-dimensional structure.

Imagine extending a line infinitely in both directions, in our three-dimensional space. Now, imagine extending another line, infinitely in both directions. Both the starting-point and the direction are chosen randomly. In this case, it would be very unlikely that the two lines "collide".

OTOH, I have read some recent theories, about the Big Bang being precisely the result of the collision of some higher-dimensional structures.