How do we know the mass of the universe

Apparently there is a visible event horizon to the universe which we cannot see past.

If we cannot be sure how many more galaxies there are (and if there may be infinitely more), how do we determine the mass of the universe?
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The quantity quoted for mass is the “observable universe”, the above statement is extrapolation on what the minimum size of the part we cant see is.

There is less need to consider anything outside of the “observable universe” in most cases and so “observable universe” is shortened to “universe.”

We don’t know the mass of the Universe. Where did you read that we do?

This is not my area of expertice obviously so if I have confused some aspects of it please excuse my ignorance, but I thought the way that we judge how the universe will end is by comparing the rate expansion with how much mass there is and how it’s affected by gravity.

You didn’t link to the source.

It’s the density that matters for the fate of the Universe, not the total mass.

Though actually, it turns out that the density of mass in the Universe doesn’t seem to matter, either, because it’s dwarfed in its effect by the dark energy (whatever the heck that is).