Universe Sandbox is a simulator not a video. While it would be possible to write a simulator to simulate what happen to get to a particular condition, it would be a whole different program, and it wouldn't simply be the same as what happened because it would only be a simulation. It would kind of be like a copy of a copy or more like a translation of a translation. Each time you slide forward then back, it would be at least slightly different. This would leave the devs supporting code to go forward, go backwards, have the two work together, so three times the work, all for a rewind that can't really tell you what happened.