Universe Sandbox shouldn't be looked at with the expectation of a game. It's exactly what it says it is, a sandbox on a universal scale, but to be used for teaching and experimentation purposes far more than simply to entertain (though perhaps some may find enjoyment over manipulating galaxies). It does an excellent job at what it attempts, but is hard to recommend to anyone but the most avid space lover for its dry and overly open ended nature.
If you're still unclear what US actually is, you can think of it as playing god with stars and planets in a galaxy of your own design. You're given complete free reign over every aspect of your universe and all that's within it, allowing you to create scenarios that could never occur in reality but nonetheless I'm sure many have wondered what would look like (for example: what if the earth was the size of the sun). It's an intriguing idea with nearly limitless options, but I have to take issue with US for doing a poor job catering to anyone but those who already posses a keen understanding of the cosmos and a fascination with what they hold. Aside from a few tutorial sections that show you a few possible things you can do, you're left to create your own fun, which for some (myself included) doesn't go much farther than shooting a dozen planet size footballs into the sun and seeing what comes of it.
I can't recommend Universe Sandbox to many, but it's not because it's bad or too much along the lines of taking an astronomy class. It knows what it is and gives you all the tools you could want to fool around in a galaxy, testing possible outcomes or teaching others about what exists in the vastness of space, but it will only ever appeal to a niche audience of astrophysicists and those that simply have a deep love of space. It's a tool that gives back whatever you put into it, the problem is just knowing how to use it.