Complex sentence: Another option is that a large mass passed through the region while it was still dense and just as a fast moving boat creates whirlpools on the surface of a smooth pond, so did this large mass leave spiral galaxies forming in its wake.
Split into simpler sentences: Another option is that a large mass passed through the region while it was still dense. Just as a fast moving boat creates whirlpools on the surface of a smooth pond, so did this large mass leave spiral galaxies forming in its wake.