The hunt for dark matter—and the associated particles and forces that we expect to accompany it—has turned up numerous false dawns over the years. Try as we might, any evidence of what makes up this invisible form of matter—thought to be the vast majority of matter in the known universe—has remained elusive. But a team of Hungarian researchers suggested in 2015 that they had found a particle, dubbed X17, that possibly interacted with dark matter in some way. Recently, in a second experiment, the team says it has fresh evidence for the X17 particle, which would change physics as we know it. But not everyone is convinced, and new experimental plans are afoot to root out the truth.