The Problems
Cosmology is the most successful theory in the history of science. The ΛCDM model describes the entire observable universe — its expansion, its structure, its composition, its history — with fewer than ten parameters. It predicts the cosmic microwave background with precision that would have been unimaginable fifty years ago. It has survived every experimental test thrown at it.
And it's incomplete in ways that are so fundamental they make the model feel like a cover story.
Here are the problems that keep cosmologists awake.
Dark Matter
About 27% of the universe is made of something that doesn't emit, absorb, or reflect light. We know it exists because it bends light from distant galaxies, holds galaxies together, and structured the early universe. We've mapped it. We've measured its mass. We've tracked its gravitational fingerprints in the CMB.
We have no idea what it is.
The leading candidate is the WIMP — the Weakly Interacting Massive Particle. A particle that barely interacts with ordinary matter, with a mass in the GeV-TeV range. We've built enormous underground detectors to catch one. We've fired the LHC at the energies where they should appear. We've looked in space for their annihilation products.
Nothing.
Axions are the other major candidate — ultra-light particles that would behave like a classical field rather than individual particles. Some experiments have found hints, but nothing confirmed.
And there are alternatives. Modified gravity theories — MOND and its relativistic extensions — try to explain galactic rotation curves without dark matter by changing how gravity works at large scales. They work at galactic scales but fail spectacularly at cosmological scales. The CMB, gravitational lensing, and structure formation all require something that behaves like massive, non-baryonic matter.
Dark matter is real. We know what it does. We just don't know what it is.
Dark Energy
The universe is expanding, and the expansion is accelerating. Whatever is causing that acceleration makes up 68% of the universe. We call it dark energy because calling it "the thing causing acceleration" would require a name longer than "dark energy" and less mystique.
The simplest explanation is the cosmological constant — a constant energy density filling space, predicted by Einstein's equations and supported by observations. But quantum field theory predicts a cosmological constant 120 orders of magnitude larger than what we measure. One hundred and twenty orders. This is the worst theoretical prediction in the history of physics.
The cosmological constant isn't the only possibility. Dark energy could be a dynamical field that evolves over time — a "quintessence" field, different from the Higgs or the inflaton but similar in spirit. It could be a sign that general relativity breaks down at cosmological scales. It could be a consequence of the multiverse — different regions of spacetime have different vacuum energies, and we live in one with a small one.
Whatever it is, dark energy dominates the energy budget of the universe. And we have no theory that explains it.
Inflation
Inflation is the theory that best explains the early universe. It solves the horizon problem, the flatness problem, the monopole problem, and it predicts the spectrum of primordial fluctuations with precision that matches observations.
But we've never observed inflation directly. We've observed its consequences. And those consequences can be described by an almost infinite number of models, each with different predictions, each compatible with current data within error bars.
Which model is right? Or is inflation itself wrong? Alternatives exist — bouncing cosmologies, emergent universe scenarios, conformal cyclic cosmology — but none comes close to matching inflation's success.
And then there's the problem of falsifiability. If inflation can predict almost anything — because different models predict different things — is it even a theory, or is it a framework so broad that it can't be wrong?
Baryogenesis
The universe is made of matter. Almost entirely matter. Antimatter is vanishingly rare. But the Big Bang should have created equal amounts of matter and antimatter. When matter and antimatter meet, they annihilate. If they'd been created equally, the universe would be nothing but photons.
Something broke the symmetry. Something favored matter over antimatter. Physicists call this "baryogenesis."
Sakharov identified three conditions needed for baryogenesis: baryon number violation, C and CP violation, and interactions out of thermal equilibrium. The Standard Model has all three, but in amounts far too small to explain the observed asymmetry. The CP violation we've measured in particle accelerators accounts for maybe one part in 10¹⁰ of the matter-antimatter imbalance.
We need new physics — new sources of CP violation, new particles, new interactions — to explain why the universe is made of stuff instead of light. We know new physics exists. We just haven't found it yet.
The Tension
Here's the problem nobody wants to talk about enough: measurements of the Hubble constant — the rate of cosmic expansion — from the early universe (CMB) disagree with measurements from the late universe (supernovae, Cepheid variables). The early universe says H₀ ≈ 67 km/s/Mpc. The late universe says H₀ ≈ 73 km/s/Mpc. The discrepancy is now at 5σ — the gold standard for "this is real."
Either there's a measurement error in one (or both), or the ΛCDM model is wrong. And if ΛCDM is wrong, everything built on top of it — dark matter, dark energy, inflation — might need rethinking.
The Bottom Line
We have a theory that works. A theory that's been tested, verified, and confirmed. A theory that describes the universe with stunning precision.
And we know it's incomplete.
Dark matter and dark energy together make up 95% of the universe. We've never detected dark matter. We don't know why dark energy exists. Inflation explains the early universe but we can't pinpoint which version is correct. Baryogenesis requires new physics beyond the Standard Model. And the Hubble tension suggests the model itself might be wrong.
The universe is mostly made of things we don't understand, described by a theory we know is incomplete, and measured by instruments that can't agree on basic numbers.
And this is the most successful science we've ever done.