Why do we presume that the universe has always expanded? When I read about the universe having always expanded from a 'singularity', I can't help but wonder if that is perhaps not the case. If a singularity is a point at which everything is infinite and is, theoretically, impossible, why are we quite so convinced it existed? What evidence is there that the universe didn't start out as a single square centimetre or even perhaps half the size it is now, because weve only known it's been expanding for the last century or so because of technological advances? Perhaps it was sizeable to begin with but the pressure of more atoms forming has pushed it outwards? Probably a ridiculous theory but Im intrigued to hear what others think about the whole subject because I just don't think the current presumption is based on any real evidence and is likely to be totally wrong.
We can, after making some approximations, trace the movements and locations of the matter in the universe back in time, using the laws of physics (mainly general relativity). 

At the moment we see a certain density of matter which is, on a large enough scale, expanding uniformly -- as if space were being blown up so that every bit of it doubles in size every seven billion or so years. 

When we extrapolate that back we find that long ago the matter density was much higher, but it was still expanding in a similar kind of way, and this would seem to hold true back to a state of very high density indeed, at which point our existing understanding of the physical laws stops giving us an extrapolation that makes any kind of sense and we have to stop.  Up to that point though, the extrapolation gives us a picture which is pretty consistent with all the various ways we have of observing it. 

So, as someone has said, the universe might have been created moments ago with everything, including our memories set up just so, but this doesn't seem a very useful line of speculation. Instead we say that, based on the physics we understand, and everything we can see and measure in the universe, it looks like its been expanding ever since it was incredibly hot and dense about 13.8 billion years ago.