The universe isn’t just expanding — it’s speeding up 13.8 billion years after the Big Bang, astronomers expected gravity to slowly slow cosmic expansion. Instead, when they looked deep into space, they found the opposite: the universe is accelerating. Whatever drives that acceleration makes up ~70% of the cosmos. We call it dark energy. We can measure it. We can see its effects. So what is it, really?

How we figured this out Cepheid stars: the distance trick Henrietta Leavitt discovered that certain stars (Cepheid variables) get brighter and dimmer with a regular period — and that period tells you their true brightness → lets us measure distance to faraway galaxies. Redshift: galaxies on the move Vesto Slipher used spectra of galaxies to show many had their light stretched to longer, redder wavelengths. Redder → moving away faster. Hubble & the expanding universe Edwin Hubble and Milton Humason combined Cepheid distances with redshift and found a pattern: >The farther a galaxy is, the faster it’s receding. That’s the Hubble–Lemaître law: clear evidence that the universe is expanding.

The shock: expansion is accelerating In the 1990s, two teams studied Type Ia supernovae, stellar explosions so consistent in brightness that they act like “standard candles.” By comparing how bright they should be to how bright they look, you can get distance. By measuring redshift, you get how fast they’re moving away. The surprise: • The supernovae were dimmer and farther away than expected. • That only made sense if, over billions of years, the universe’s expansion had sped up instead of slowing down. This cosmic acceleration is what we now attribute to dark energy.

We Call the Mystery “Dark Energy” We give this unknown cause a placeholder name: dark energy. It behaves like a repulsive component of the cosmos, pushing space itself apart. By our best measurements, the universe is roughly: • 5% normal matter (stars, planets, us) • 27% dark matter • 68–70% dark energy — the driver of acceleration.

What Could Dark Energy Be? Cosmologists are testing a few main ideas: • Vacuum energy (cosmological constant): empty space has a tiny built-in energy that pushes it apart. • Quintessence: a new, dynamic energy field spread through the universe. • Wrinkles in spacetime: defects such as cosmic strings from the early universe. • Modified gravity: maybe Einstein’s equations need a tweak on the largest scales.

Where we really are In our best models, dark energy acts like a kind of negative pressure, pushing galaxies apart and nudging the universe toward a cold, stretched-out “Big Freeze.” • We see its effects in supernovae, galaxies, and the cosmic microwave background • We can measure its share of the cosmos (~70%) • We still don’t know what it is Dark energy is just our label for that mystery – the unknown that quietly decides how the universe ends.