COSMAWATCH

The Missing Mass Problem

In the 1970s, astronomer Vera Rubin and her colleague Kent Ford measured how fast stars orbit around the centres of spiral galaxies. Based on the visible mass of the galaxy, stars further from the centre should orbit more slowly — just as outer planets orbit the Sun more slowly than inner ones (Kepler's laws). Instead, Rubin found that the rotation curves were nearly flat: stars at large radii orbit just as fast as those near the centre.

This means there must be far more mass than what's visible. This unseen mass — dark matter — doesn't emit, absorb, or reflect light, making it invisible to all our telescopes. Yet its gravitational effects are unmistakable. Subsequent evidence piled up: gravitational lensing of galaxy clusters, the structure of the CMB, and computer simulations of galaxy formation all point to enormous quantities of dark matter.

Current estimates suggest dark matter makes up about 27% of the total energy content of the universe, while ordinary (baryonic) matter — everything made of protons, neutrons, and electrons — accounts for only about 5%. The nature of dark matter remains one of the greatest mysteries in physics.

Dark Energy and the Accelerating Universe

In 1998, two independent teams studying Type Ia supernovae as 'standard candles' made a startling discovery: distant supernovae were dimmer than expected — they were further away than a decelerating or steady expansion would predict. The expansion of the universe is not slowing down due to gravity — it is accelerating.

The cause of this acceleration is dubbed dark energy, and it accounts for about 68% of the total energy content of the universe. It behaves like a 'cosmological constant' — an energy inherent to empty space itself, as Einstein once proposed and then famously retracted, calling it his 'greatest blunder.' It now appears he was onto something.

The nature of dark energy is completely unknown. It might be the energy of the quantum vacuum, a slowly evolving field (quintessence), or something else entirely. The 2024 results from the DESI survey began to hint that dark energy might not be constant — but the story is still unfolding.

The Cosmic Pie Chart

Putting it all together: the universe we can see and touch — stars, planets, gas, dust, you, everything — is just about 5% of the total. About 27% is dark matter we can detect only by gravity. Around 68% is dark energy driving the accelerating expansion. We understand the 5% rather well. The remaining 95% we understand barely at all.

This is not a failure but a triumph: we have precisely quantified our ignorance. The measurements are exquisitely consistent across completely independent methods. Now the task is to understand what dark matter and dark energy actually are — arguably the most significant open challenge in all of physics.