Originally posted by @sonhouse
Here is a professional astronomer, Phd, what he said:
"What is the mass of the Universe? (Intermediate)
What is the mass of the Universe? Also how can you prove that this is the true mass of the Universe?
As nobody knows the size of the universe, one cannot really talk about the mass of the universe, though one can talk about the mass of the observabl ...[text shortened]... t the observable universe. And I think he didn't take into account dark matter, not sure though.
We know that our universe is *not* infinite in size, right? We know what the mean density of the universe that we see, right? And we know that the universe has a diameter of at least 93 billion light years, then we know the minimum limit of how much mass there is in the universe.
Thomas Pilgaard says:
"... the 46 billion light years figure is the current radius of the observable universe (giving it a diameter of ~93 billion light years). 46 billion light years in either direction are objects that are currently at that distance of 46 billion light years away, but whose light has just reached us after being emitted 13.8 billion years ago."
Frank Heile, Ph.D. in Physics from Stanford University says
"So, the diameter of 93 billion light years is, at most, a theoretical estimate of the current distance of all the matter that we can NOW see, even if the light we see is 13.8 billion years old."
This is what we know now. I'm sure that we will eventually find out the magnitude of the inflation just after t=0, so the true size of our universe is not at all unknowable, if you ask me.