Big Bang
The Big Bang is our best account of how the universe began: about 13.8 billion years ago everything we can see was packed into a state unimaginably hot and dense, and it has been expanding and cooling ever since. Picture the dots drawn on a balloon as you blow it up — every dot drifts away from every other, not because the dots are moving across the rubber, but because the rubber itself is stretching. The universe is doing the same thing, on the grandest scale.
Despite the name, it was not an explosion that flung matter outward into empty space. Space itself was expanding — there was no "outside" for it to expand into, and no single spot you could point to and call the center. Every galaxy sees all the others rushing away from it, which is exactly what you'd expect if space everywhere is swelling.
We're not just guessing. The whole sky glows faintly with leftover heat from that hot beginning — a feeble microwave hum coming from every direction — and the way galaxies fly apart, faster the farther off they are, fits the picture beautifully. Rewind that expansion in your mind and everything crowds back together to a single fiery start.
The name comes from astronomer Fred Hoyle, who used it in a BBC radio talk on 28 March 1949 while contrasting this picture with his own rival "steady-state" model. Hoyle was a critic of the theory, and the label is often said to have been a put-down — though Hoyle himself insisted he never meant it that way, only as a vivid image to help listeners feel the difference between the two ideas. Either way, the name stuck and became the standard term.