What is dark matter ?
All the ordinary matter we can find accounts for only about 4 percent of
the universe. We know this by calculating how much mass would be needed
to hold galaxies together and cause them to move about the way they do
when they gather in large clusters. Another way to weigh the unseen
matter is to look at how gravity bends the light from distant objects.
Every measure tells astronomers that most of the universe is invisible.
It's tempting to say that the universe must be full of dark clouds of
dust or dead stars and be done with it, but there are persuasive
arguments that this is not the case. First, although there are ways to
spot even the darkest forms of matter, almost every attempt to find
missing clouds and stars has failed. Second, and more convincing,
cosmologists can make very precise calculations of the nuclear reactions
that occurred right after the Big Bang and compare the expected results
with the actual composition of the universe. Those calculations show
that the total amount of ordinary matter, composed of familiar protons
and neutrons, is much less than the total mass of the universe. Whatever
the rest is, it isn't like the stuff of which we're made.
The quest to find the missing universe is one of the key efforts that
has brought cosmologists and particle physicists together. The leading
dark-matter candidates are neutrinos or two other kinds of particles:
neutralinos and axions, predicted by some physics theories but never
detected. All three of these particles are thought to be electrically
neutral, thus unable to absorb or reflect light, yet stable enough to
have survived from the earliest moments after the Big Bang.
What is dark energy ?
Two recent discoveries from cosmology prove that ordinary matter and
dark matter are still not enough to explain the structure of the
universe. There's a third component out there, and it's not matter but
some form of dark energy.
The first line of evidence for this mystery component comes from
measurements of the geometry of the universe. Einstein theorized that
all matter alters the shape of space and time around it. Therefore, the
overall shape of the universe is governed by the total mass and energy
within it. Recent studies of radiation left over from the Big Bang show
that the universe has the simplest shape—it's flat. That, in turn,
reveals the total mass density of the universe. But after adding up all
the potential sources of dark matter and ordinary matter, astronomers
still come up two-thirds short.
The second line of evidence suggests that the mystery component must be
energy. Observations of distant supernovas show that the rate of
expansion of the universe isn't slowing as scientists had once assumed;
in fact, the pace of the expansion is increasing. This cosmic
acceleration is difficult to explain unless a pervasive repulsive force
constantly pushes outward on the fabric of space and time.
Why dark energy produces a repulsive force field is a bit complicated.
Quantum theory says virtual particles can pop into existence for the
briefest of moments before returning to nothingness. That means the
vacuum of space is not a true void. Rather, space is filled with
low-grade energy created when virtual particles and their antimatter
partners momentarily pop into and out of existence, leaving behind a
very small field called vacuum energy.
That energy should produce a kind of negative pressure, or repulsion,
thereby explaining why the universe's expansion is accelerating.
Consider a simple analogy: If you pull back on a sealed plunger in an
empty, airtight vessel, you'll create a near vacuum. At first, the
plunger will offer little resistance, but the farther you pull, the
greater the vacuum and the more the plunger will pull back against you.
Although vacuum energy in outer space was pumped into it by the weird
rules of quantum mechanics, not by someone pulling on a plunger, this
example illustrates how repulsion can be created by a negative pressure.
