20 years ago, our understanding of the Universe underwent a revolution. For generations, we had known the Universe was expanding, but we didn’t know its fate. Whether it would recollapse (with gravity defeating the expansion), expand forever (with the expansion defeating gravity), or live right on the border between the two cases (with expansion and gravity perfectly balanced) was one of cosmology’s greatest open questions.

Then, in 1998, two independent teams — the high-z supernova search team and the supernova cosmology project — both released their results that showed that ultra-distant supernovae were far too faint to be consistent with any of these. The Universe wasn’t just expanding, the expansion was accelerating. Expansion defeats gravity, and a new form of energy was required to explain the observations: dark energy.

But many scientists were skeptical. After all, if things were fainter than expected, maybe the Universe wasn’t accelerating. Maybe it was just dust? For years, that notion was the main competing idea to dark energy. Here’s how it died.

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