big bang was expansion of space NOT an explosion into space - Matt Strassler

http://profmattstrassler.com/articles-and-posts/relativity-space-astronomy-and-cosmology/history-of-the-universe/big-bang-expansion-not-explosion/

Fig. 1: What the Big Bang was not: An explosion of a seed into a pre-existing space. The explosion is created by a process that generates tremendous heat and pressure inside the seed, which rushes outward as a ball of hot material exploding into the pre-existing space. The Big Bang is nothing like this.

Fig. 2: The expansion of space is depicted by the growth of the distance between the grid lines. Normal objects remain of the same size, but the distance between them grows as the space uniformly expands.  This type of expansion can occur either in a hot or cold universe, under the right conditions, and it can be very rapid.