How to store BIG int values

For values which only require up to 28 digits, you can use System.Decimal. The way it's designed, you don't encounter the scaling issue you do with double, where for large numbers the gap between two adjacent numbers is bigger than 1. Admittedly this looks slightly odd, given that decimal is usually used for non-integer values - but in some cases I think it's a perfectly reasonable use, so long as you document it properly.

For values bigger than that, you can use System.Numerics.BigInteger.

Another alternative is to just use double and accept that you really don't get a precision down to the integer. When it comes to the distance between galaxies, are you really going to have a value which is accurate to a metre anyway? It does depend on how you're going to use this - it can certainly make testing simpler if you use a nicely-predictable integer type, but you should really think about where the values are going to go and what you're going to do with them.