Scientists said they found a subatomic particle resembling the long-elusive Higgs boson, a landmark discovery that could explain why particles have mass and, by extension, why stars, planets and all other objects in the universe exist at all.

On Wednesday morning, hundreds of scientists assembled at the European laboratory CERN in Geneva, and many more tuned in to a live webcast to hear how fresh data from the Large Hadron Collider had conclusively revealed the existence of...