Dark Matter, Dark Energy and How the Universe Ends

Add up every star, every planet, every cloud of gas and every black hole in the observable universe. All of it together comes to roughly 5% of what is actually there.

About 27% is dark matter. About 68% is dark energy. Neither has ever been directly detected. Both are inferred entirely from their gravitational effects, and they are not variations on the same idea — they behave in opposite ways and were discovered by completely different routes.

This is not a comfortable position for a science to be in, and it is worth being honest about that. But the evidence that something is there is unusually strong, and it comes from independent directions that agree with each other.

Dark matter: galaxies spinning too fast

In the 1930s, Fritz Zwicky measured how fast galaxies were moving within the Coma Cluster and found they were moving far too quickly for the cluster's visible mass to hold them together. The cluster should have flown apart long ago. He proposed unseen matter and was largely ignored for forty years.

The case became difficult to dismiss in the 1970s, when Vera Rubin and Kent Ford measured galaxy rotation curves — how orbital speed varies with distance from a galaxy's centre.

The expectation was straightforward. In the solar system, where nearly all mass is concentrated in the Sun, outer planets orbit more slowly than inner ones: Neptune crawls, Mercury races. Galaxies also have most of their visible mass concentrated centrally, so stars in the outskirts should orbit more slowly.

They do not. Rotation curves stay flat far beyond the visible edge of the galactic disk. Stars at the outskirts move about as fast as stars much closer in, which requires far more mass than we can see, distributed in an extended halo rather than concentrated in the middle.

Why it is probably not just faint ordinary matter

The obvious first explanation is dim ordinary matter — cold gas, faint stars, rogue planets. Several independent lines of evidence rule this out.

Big Bang nucleosynthesis. The relative abundances of hydrogen, helium and lithium produced in the first few minutes depend sensitively on how much ordinary matter existed. The observed abundances match a universe with roughly the amount of ordinary matter we can already account for — leaving no room for a large hidden reservoir of it.

The cosmic microwave background. The detailed pattern of temperature fluctuations in the CMB depends on how matter clumped in the early universe. Ordinary matter interacts with light, which resists clumping; dark matter does not, so it can begin collapsing earlier. The observed pattern requires both components in specific proportions, and those proportions are measured to high precision.

The Bullet Cluster. Two galaxy clusters collided. The hot gas — most of the ordinary matter — collided too, slowed down, and piled up in the middle. But gravitational lensing shows the mass passed straight through and now sits ahead of the gas on both sides. Mass and visible matter physically separated. That is difficult to explain by modifying gravity, and straightforward if most of the mass is a substance that barely interacts with anything.

What dark matter actually is remains unknown. WIMPs were the leading candidate for decades, and increasingly sensitive direct-detection experiments have found nothing, steadily shrinking the available parameter space. Axions are now a major focus. Primordial black holes remain possible within limited mass ranges. This is an active and genuinely unresolved problem.

Dark energy: expansion that speeds up

Dark energy arrived from a completely different direction, and it was not something anyone was looking for.

By the 1990s the universe was known to be expanding. The open question was how much that expansion was slowing under gravity — enough to eventually reverse, or not quite. Two teams set out to measure the deceleration using type Ia supernovae as standard candles.

In 1998 both teams found the same thing, and neither believed it initially. Distant supernovae were fainter than expected, meaning further away than a decelerating universe would place them. The expansion is not slowing. It has been accelerating for roughly the last five billion years. The result won the 2011 Nobel Prize in Physics.

Something is pushing space apart, and its influence grows as the universe expands, because as matter thins out its density stays roughly constant while matter's density falls. The simplest description is a cosmological constant — an intrinsic energy of empty space itself.

Here the honesty is uncomfortable. When quantum field theory is used to estimate the energy of the vacuum, the answer disagrees with the observed value by something in the region of 120 orders of magnitude. This is routinely described as the worst quantitative prediction in the history of physics. We have a description that fits the data and no accepted explanation of why the number takes the value it does.

How it ends

The fate of the universe depends on how dark energy behaves over cosmological time, and the honest summary is that we do not yet know well enough to choose between the scenarios.

Heat death — the current default

If dark energy is a true constant, expansion continues forever at a steady rate. Galaxies outside our local group recede beyond the horizon and vanish from view. Star formation exhausts the available gas. The last stars — small red dwarfs, which burn extraordinarily slowly — fade after perhaps 100 trillion years.

What follows is a long darkness of white dwarfs cooling, neutron stars cooling, and black holes slowly evaporating via Hawking radiation over timescales up to 10^100 years. Eventually there is nothing but sparse radiation at nearly uniform temperature, with no usable energy gradients anywhere. Nothing can happen, because everything that happens requires a difference in temperature. This is the heat death, and it is the mainstream expectation.

The Big Rip

If dark energy's strength increases over time — described by so-called phantom energy — the acceleration becomes progressively more violent. Galaxy clusters are torn apart, then galaxies, then solar systems, then planets, then atoms, in a cascade ending in a finite time. Current data do not favour this, but do not entirely exclude it either.

The Big Crunch

If dark energy weakens or reverses, gravity could eventually win, expansion could halt, and everything could collapse back together. This was the leading scenario before 1998. It is now strongly disfavoured, though some dynamical dark energy models keep it technically alive.

Vacuum decay

The most unsettling option, and one that follows from a specific reading of the measured Higgs and top quark masses: our vacuum may not be the lowest-energy state available, only a long-lived metastable one. If so, a bubble of true vacuum could nucleate somewhere and expand at light speed, rewriting physics inside it.

There would be no warning of any kind, since the bubble wall travels as fast as any signal announcing it. Current estimates put the expected timescale far beyond the age of the universe, which is the only reassuring thing about it.

Why the next decade matters

This is not a settled field waiting for textbooks to catch up. It is under active pressure from new data.

The Dark Energy Spectroscopic Instrument has been mapping tens of millions of galaxies to trace expansion history in detail, and its early results have hinted that dark energy may not be perfectly constant after all — a finding that, if it holds up with more data, would be genuinely significant. ESA's Euclid mission, launched in 2023, is surveying a third of the sky to measure dark matter's distribution through gravitational lensing. The Vera C. Rubin Observatory will image the entire visible sky repeatedly, gathering supernovae in numbers that dwarf the 1998 samples.

The 95% problem is likely to look different in ten years than it does now. That is the appeal of it.