There’s an enormous controversy in astrophysics today over how quickly the Universe is expanding. One camp of scientists, the same camp that won the Nobel Prize for discovering dark energy, measured the expansion rate to be 73 km/s/Mpc, with an uncertainty of only 2.4%. But a second method, based on the leftover relics from the Big Bang, reveals an answer that’s incompatibly lower at 67 km/s/Mpc, with an uncertainty of only 1%. It’s possible that one of the teams has an unidentified error that’s causing this discrepancy, but independent checks have failed to show any cracks in either analysis. Instead, new physics might be the culprit. If so, we just might have our first real clue to how dark matter might be detected.