This may sound like a noob question, even more if it's asked by a undergraduate physics student, but here it's: How do we know about the existence of 100 billion galaxies out there (even that it's an average value)? I mean, how do we know about these exact number? Why not, say, 200 billion or 50 billion? Is there a concrete way to obtain the value out of the Relativity Theory? If not, what theory should we use to get 100 billion?