Something invisible is pushing the universe apart faster and faster, and no one knows what it is

For most of the 20th century, astronomers assumed gravity was gradually slowing the expansion of the universe after the Big Bang. Observations of distant supernovae in the late 1990s showed the opposite: the expansion is speeding up.

To explain this, physicists introduced the concept of "dark energy" — a force, or a property of space itself, that pushes galaxies apart. Current estimates put it at roughly 68% of the total energy content of the universe.

The problem is that "dark energy" is, in practice, a name given to an observed effect, not a description of a mechanism — nobody knows whether it's a cosmological constant baked into the fabric of the vacuum, some unknown energy field, or a sign that our understanding of gravity at the largest scales is incomplete.

That means most of the energy in the universe — once you add in dark matter — remains, quite literally, unknown to science, even though we can measure its effects with great precision.