A faint microwave glow fills all of space — the afterglow of the Big Bang, the universe's baby picture. Reading it, plus the way galaxies move, reveals that most of the cosmos is dark matter and dark energy we can't yet see.
The universe's baby picture
About 380,000 years after the Big Bang, the cooling universe let light travel freely for the first time. That ancient light, stretched by expansion into microwaves, still bathes the whole sky as the cosmic microwave background (CMB). Its near-perfect uniformity — with tiny fluctuations that seeded galaxies — is powerful evidence that the universe began hot and dense.
Galaxies rotate too fast for their visible mass to hold them together — something unseen adds gravity: dark matter, roughly five times more abundant than ordinary matter. And in the 1990s, supernova surveys revealed the expansion is accelerating, driven by a mysterious dark energy. Together they make up about 95% of the universe.
The standard model, ΛCDM, weaves these together: ordinary matter, cold dark matter, and a cosmological constant (Λ) for dark energy. It fits the CMB, galaxy surveys, and supernovae remarkably well — even as the fundamental nature of the dark components remains one of physics' deepest open questions.
- Estimate a spiral galaxy's mass from its light (its visible stars).
- Estimate its mass again from its rotation speed using orbital dynamics (faster orbits at a given radius imply more enclosed mass).
- Compare the two figures. Which is larger, and by roughly how much?
- State what the discrepancy implies and name the leading explanation.
What you should see: You reproduced the classic rotation-curve argument: the dynamical mass vastly exceeds the luminous mass, pointing to dark matter.