How we noticed
The light from distant galaxies turned out to be shifted toward the red end of the spectrum, and the farther away a galaxy is, the stronger the shift. This happens when a source is moving away. But galaxies aren't moving away like fragments from an explosion: the distance between them is increasing, and the light stretches along the way too.
This led to the rule discovered in the late 1920s: the speed of recession is proportional to distance. Twice as far means twice as fast. No explosion behaves this way, but uniformly stretching space does.
Imagine raisin dough in the oven. The dough rises, and each raisin sees all the others moving away from it. None of them is at the center.
The explosion picture stays in our heads largely because of the name. It was coined by an astronomer who actually rejected the theory, and he said it mockingly. The name stuck, along with an image that isn't part of the theory itself.
There was an explosion at one point, and matter flew from it into empty space.
Space didn't surround the event; it took part in it. Expansion is happening everywhere at once, so it has no center, edge, or empty space outside it.
What expands and what doesn't
Distances grow only where nothing is holding things together. Inside a galaxy, gravity is stronger than expansion, so the Solar System isn't stretching, Earth isn't swelling, and a ruler isn't getting longer. The gaps between distant clusters grow, where gravity is already weak.
In the late 1990s, something unsettling became clear: expansion isn't slowing down, as expected, but accelerating. Its cause was called dark energy, but the name describes an observation, not an understanding. No one can yet say exactly what creates it.
The question "why is it expanding?" is trickier than it seems. The equations of gravity don't allow a static universe: it has to either expand or contract. Observations show that it is expanding. But what gave it its initial push remains an open question.
There is a practical consequence too. If the acceleration continues, distant galaxies will eventually pass beyond our observable horizon, and the sky of future observers will be noticeably poorer than ours. We live in a fortunate era for astronomy, and that's not just a figure of speech.
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Sources
- The relationship between distance and recession speed: observations by Edwin Hubble and Vesto Slipher, 1910s-1920s. Accelerated expansion was discovered in 1998 through observations of supernovae