Well, if you want some really basic calculations. Just remember these are extremely rough calculations, and I mean really rough, but at the very least I should get the scale of how powerful a computer you'd need.
On my system, I have a gtx 970 and an i7 4970k. When I run the solar system, it sits at a stable 144 fps. When I have 2 solar systems, it drops to about 30 fps. Assuming that every solar system in the universe is on average identical to ours, then you'd need a computer about 150 billion times more powerful than mine to run at 30 fps to generate our galaxy (Just the solar systems) at the level of detail Universe Sandbox ² provides at 15 days per real-time second. This is assuming that the scaling of the fps drop is linear, too.
Putting that into perspective, you'll usually hear how our current computers are hundreds of millions of times faster than the supercomputers they used at NASA during the moon launch.
Even our fastest supercomputer at the moment of this post, the Sunway TaihuLight at 93 Petaflops, wouldn't be able to run a galaxy generated with Universe Sandbox ². If we assume that flops is the absolute way to measure computing power and that our computers and super computers work the same way (Which it isn't and they don't, but let's just assume it is), then my computer is about 5 teraflops, so in terms of raw calculation, the Sunway TaihuLight is only 186,000 times faster. Nowhere near enough.