|MadSci Network: Astronomy|
Your question is one of the really big questions. I don't know if I would call it unfair, but it is at the very edge of what we can scientifically answer. It may indeed fall more easily into the realm of philosophy or what some call metaphysics.
The Observable Universe has some mass, let's just refer to its value as M, because we currently don't know just what the value is. Through scientific investigation we will one day be able to determine what the value is. But as to the question of why it is what it is, well, that is not so straightforward to answer. One answer would be that everything that occurs in the Universe happens by chance. So, the mass that the Universe ended up with after its initial primordial fireball phase (when it was very dense, hot, and consisted of matter and energy in equilibrium) was just a cosmic accident. No rhyme or reason. There is another conjecture on the nature of the Universe which postulates that ours is but one Universe in a large, perhaps even infinite, ensemble of Universes existing in some greater multiverse. Each Universe is separate from one another and the ensemble spans all possible characteristics that a Universe could possibly have in this multiverse. So, here in our Universe, we have the mass we have because that's the one we could have. Other Universes probably have different masses.
Arguments for or against these conjectures are usually made from philosophical principles known as anthropic. The main idea being that in order for us to be able to ask these questions the Universe had to be one in which we could exist in the first place. The questions can go much deeper than just, "why do we have the mass that we do". In the study of Cosmology we find that our theories require the laws of nature and the sequence of cosmic evolutionary events to be rather fine-tuned in order for the Universe to have ended up in a state where beings such as ourselves could even exist. Thus we find ourselves putting forth conjectures based on anthropic arguments. These conjectures may be beyond the ability of experimentation. If that is so then science cannot answer the question of "why". Our level of uncertainty is such that we cannot even be sure at this point whether these are questions that science can answer.
For reference I would suggest reading some of the material on the following website: Anthropic-Principle.com. It is maintained by a philosophy lecturer from Yale University.
I hope that helps a little. Enjoy thinking those deep thoughts. I certainly get a kick out them.
UC, Berkeley Astronomy Department
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