I love this, but there is one thing preventing me from using it... The sandbox is good to map a single orbit, but when you start adding orbits on other orbits the simulator moves to a crawl. At the same time, the sandbox is not even maxing out a single core, and is using around 17% of the CPU...
Moving from a Q9550 -> i7 2600K changed the Max CPU from 25%->17%.. Simply put, we need more threads for calculations.
Also you should be able to create an system, and then add that system to a larger system. By putting something in orbit, and then adding gravity pull by (for example) adding a moon, the auto orbit of the first item is now pulled by the moon, and the moon pulls the planet. Ultimately, both the moon and the planet get pulled by the star, and the item then breaks orbit and moves slowly towards the star.
I want to see Dark Matter, as I can not create true galaxy's as these large objects fall apart under newtons law. Adding more matter to the black hole in the center does not fix this, it simply causes everything to fall into the black hole. The matter has to be spread out. There is allot of information and data on this, but the calculations are huge. I think you would need to use the video card for a large amount of these calculations. You can easily run 500 threads at once.
I would also like to see the effects of black holes, including quasars and other behaviors. These behaviors are well known and really well documented.
Ultimately you have done a great job. I am amazed that NASA has not released this. I think Open Source solution would provide allot of opportunity for some amazing improvements. I would suggest you open up the code and let people know about it, I think people will go nuts for this and so many advanced physics could be added, including system optimizations. Its too much for a few people too do. Those that want to support the project will... Keep it on steam, and continue to sell it. If people want to download it for free, then so be it.