The universe is expanding because of the Big Bang explosion, right?
Not exactly, depending on what you mean by explosion. We tend to use the word explosion to mean a local event, occurring at a particular point, whose pressure is much greater than that of the surrounding and so causes material to expand rapidly away from the center. This picture fails in two ways for the Big Bang. First, there is no center point; the expansion happens everywhere at once. Secondly pressure doesn’t cause the expansion because there is no lower pressure region surrounding the universe; there is nothing to expand into. In fact, in general relativity pressure is a form of energy that effectively acts as mass and the gravitational attraction on this extra mass retards the expansion. One can think of the overall curvature of spacetime as causing the expansion.
