The answer to the enduring question of the smallest thing in the universe has evolved along with humanity. People once thought grains of sand were the building blocks of what we see around us. Then the atom was discovered, and it was thought indivisible, until it was split to reveal protons, neutrons and electrons inside. These too, seemed like fundamental particles, before scientists discovered that protons and neutrons are made of three quarks each.

“This time we haven’t been able to see any evidence at all that there’s anything inside quarks,” said physicist Andy Parker. “Have we reached the most fundamental layer of matter?” And even if quarks and electrons are indivisible, Parker said, scientists don’t know if they are the smallest bits of matter in existence, or if the universe contains objects that are even more minute.