I just downloaded this simulation game called Universe Sandbox off of Steam. This allows you to create universes, modify them, and see what happens. For $4, it's something to look into.
I got it and tinkered around a little with it a while back. For the money it's well worth it even if you don't get into it too much and from what I gather there's a lot of opportunity to tinker around with details.
I got that through Steam, and it's awesome. Another writerly use for it: Figuring out distances between systems, getting a visual hold on galactic "real estate" for far-flung space opera, and gaining a better appreciation for the "inky void" and just how vast it is.