How did the big bang theory propose spreading out from the center?

Answer 1

Your question shows a basic misunderstanding of the Big Bang, common even amongst scientists and fostered by popular mis-representations.

Our Universe did not begin with an expansion from a center point. In particular, it was not an explosion of matter into empty space. Instead, it was an expansion of space itself, in which matter came along, becoming less dense as space expanded. There was no center point from which space expanded; all points in our Universe expanded away from all other points in exactly the same way. If this were not the case, then the strenfth of the cosmic background radiation would have a preference in direction. Instead, the CBR is isotropic to one part in 10,000.

The best way to envision our expanding Universe -- and even this way has its flaws -- is to think of space as the surface of an expanding balloon, with matter and galaxies like ink dots on this surface. You'll notice that no point on this balloon's surface has any right to claim itself as the center of the expansion; all points are simply moving away from each other.

The problems with THIS viewpoint is that space (ie, the balloon surface) is viewed as (1) expanding INTO something, (2) two-dimensional, and (3) curved -- whereas space in our Universe is instead not expanding into anything, three-dimensional, and (as best we can tell) mathematically "flat."

If your actual question is "Why did space start to expand about 13.7 billion years ago?", the answer is that we don't know yet. We're pretty much like Johannes Kepler when he observed that planetary orbits were eliptical -- we know this to be true, we just haven't found out what's causing it.

Answer 2

The Big Bang theory proposes that the universe began expanding from an extremely hot and dense state about 13.8 billion years ago. As the universe expanded, matter spread out uniformly in all directions from a central point. This expansion continues today, with galaxies moving away from each other due to the overall expansion of the universe.