Complex sentence: If one of the colliding galaxies is much larger than the other, it will remain largely intact after the merger; that is, the larger galaxy will look much the same while the smaller galaxy will be stripped apart and become part of the larger galaxy.
Split into simpler sentences: If one of the colliding galaxies is much larger than the other, it will remain largely intact after the merger. The larger galaxy will look much the same, and the smaller galaxy will be stripped apart and become part of the larger galaxy.