>>10874691well the universe isn't flat. it's slightly hyperbolic. very very slightly hyperbolic -- basically translates to the idea that the universe's expansion is accelerating and that things are all getting a bit farther away all the time and they keep going away from one another faster and faster. but flat is a fine approximation. (this is why we say the universe is almost Minkowskian but actually de-Sitter)
for your actual question, then the answer is yes in standard cosmology. it's hard to think of a good analogy but let me make two different ones. first, the balloon anaology. imagine we had a balloon with dots drawn on it. the dots are galaxies in a two-dimensional universe. now if we blow the balloon up, they all get further from one another for an ant who is stuck to crawling on the surface. that's what the universe expanding means. but imagine you sucked all the air out of the balloon so all the dots came together at the same place. then eventually the ant would be standing on every point in the entire universe. that's sort of like the big bang. in other words, all the points of the universe were in the same place at the time of the big bang
second analogy is a little more abstract. imagine you have the real number line and you want to calculate how far any two points on it are. to start with, for points A and B, the distance is B-A. now imagine we had some factor c called the "scale factor" where the distance actually changes to c(B-A). okay? so if the scale factor is 4 then the distance between A and B is 4(B-A) i.e. the universe looks 4 times bigger.
now imagine that c is some linearly increasing thing over time. in fact in the real universe, things are expanding so c is increasing. now imagine that c is a linear function of time. c=k*t. what if t=0? then the distance between _any_ two points A and B is 0, no matter what points you pick. that is like the big bang.
