Standard big bang theory states that the amount of space in the universe is increasing, but the amount of matter is fixed.
If you take any amount of matter and compress it into a single point, that point becomes infinitely dense (and infinitely hot). That was what was supposed to have been the state of the universe at the time of the big bang. However, the mathematical formulas for understanding the laws of nature don't work when one of the starts putting in "infinity". You get all kinds of nonsense results. So, really smart physicists have come up with all kinds of theories and speculations as to how to tweak the model in order to make the impossibility of the big bang infinity work.
Now, a few billion years after the big bang, scientists observe that everything in the universe is moving away from everything else. The common analogy is to describe it as a loaf of yeasted raisin bread rising. The raisins are the matter in the universe and the bread is the empty space. As the empty space increases (the bread rises) the raisins move further away from each other. The number of raisins (amount of matter) remains the same, but the size of the bread (universal empty space) increases.
Then you get into what and where the matter in the universe is. 99% of it is this mysterious dark matter or dark energy that no one knows what it is and no one can detect, but must be there to make the mathematics work.
So, all in all, the physicists have no clue, they are just guessing wildly. Check out Carana Renu??(TM)s blog, a good friend of mine who has a PhD in astrophysics.
As I mentioned in this blog posting of mine: looking into the 5000-year old Vedic literature of ancient India sheds some light on the mystery. Those writings state that there are an unlimited number of parallel universes, each finite in the amount of matter they contain. Within each universe there are roughly 36000 cycles of partial creations and destructions (one might call them big bangs and big crunches) before one particular universe is completely destroyed (after 311 trillion years). Within the universe humans occupy only a tiny amount of space. The Vedas state that there are 14 different loka-systems (literally: places). Earth and reality as we see it occupies only one of these. The most advanced living beings in this universe live in a place called Satyaloka (literally: perfected-place) (and yes indeed, human beings are not the pinnacle of evolution). The big bang doesn't kill the people in that supreme sphere of existence. They only die after 311 trillion years (lifetime of Brahma) when the actual universe is destroyed (sucked into a skin pore of Maha-Vishnu).
So, from the Vedic perspective, it is no wonder that the Universe doesn't make sense to the scientists, since most of it is in different (higher-dimensional?) space which we can't understand with our limited, low-class material bodies.
... and I have not even mentioned the spiritual reality where time does not exist.