Dark matter map reveals ‘invisible cosmic web’

Clumps and tendrils: A visualisation of how dark matter is distributed across the cosmos.

The things we can see account for less than 20% of the universe’s mass. Almost nothing is known about the rest. Now, a huge project is attempting to map how ‘dark matter’ is distributed.

From the stars in the sky to the dirt on your shoes, everything we can see is composed of particles like atoms. This is ‘matter’, and until recently scientists believed that everything in the universe was made of it.

They were wrong. In 1933, astronomers watching distant galaxies noticed that the mass of their stars was not enough to hold them together. Some unseen thing was exerting a huge gravitational pull. That mysterious ‘something’ became known as ‘dark matter’.

Today, physicists accept that dark matter is all around us: it accounts for nearly 85% of the universe’s mass. But beyond that we know almost nothing. Because it does not reflect light, dark matter is invisible. What’s more, it scarcely reacts with other particles: collisions between entire galaxies leave dark matter unscathed.

Little by little, however, astronomers are penetrating the gloom. The Dark Energy Survey is an ambitious project to map the distribution of dark matter across the cosmos by measuring the minuscule distortions it causes in light from faraway stars. This week, the project released its first results.

The data shows that ‘clumps’ of dark matter are concentrated amid galaxies, connected by spindly ‘tendrils’. It is a valuable insight into a mystifying phenomenon. But what the scientists hope to eventually unearth is more baffling still.

Stars, planets and everything we think of as ‘stuff’ makes up only 0.4% of the universe. A further 3.6% is composed of particles of gas floating between galaxies. Dark matter accounts for 27%. The remaining 69% is thought to be composed of something known as ‘dark energy’. Even less is known about it than dark matter — the only reason scientists suspect its existence is that something enormously powerful must be forcing the universe to carry on expanding.

By tracking the rate at which tendrils of dark matter are lengthening, scientists at the Dark Energy Survey hope to gain insights into the force acting upon them. They may find a way to measure Dark Energy. Or they may find that it doesn’t exist at all; that would cast doubt on the foundations of modern physics.

Unknown unknowns

Ever since the dawn of the enlightenment three centuries ago, scientists have shattered the boundaries of human knowledge: we know about particles so tiny that they will never be perceived and galaxies so distant that even light takes 13 billion years to reach them. Some believe that no truth is beyond the reach of science.

Yet the more we understand about the universe, the more mysteries our searches reveal: discoveries like dark matter relegate the world we see and experience ever further in significance. Perhaps there are things that we can simply never know.

You Decide

  1. Are there mysteries that science can never resolve?
  2. Do discoveries in physics have any effect on the way you see the world? Should they?

Activities

  1. In groups, try to come up with an analogy that explains dark matter and why scientists believe it exists.
  2. Scientists first speculated about the existence of dark matter because it could explain why galaxies were not flying apart and disintegrating. Briefly explain in your own words why this would happen without dark matter.

Some People Say...

“There are more things in heaven and Earth than are dreamt of in your philosophy.”

Shakespeare

What do you think?

Q & A

If I can’t see dark matter and can never be sure that it even exists, why should I care about it at all?
It may seem like a bizarre, speculative abstraction now, but so do many discoveries that eventually change the world in powerful ways. Einstein’s famous theory of mass-energy equivalence, for instance — the one usually expressed as E=mc² — was totally theoretical until it was used to create nuclear power. Even everyday objects like fridges rely on discoveries that initially seem to have no practical application.
So what will dark matter do for us?
It’s impossible to say for now. But even if discoveries like this serve no practical purpose, they expand our understanding of the universe we live in and how it works. Isn’t that valuable enough in itself?

Word Watch

Atoms
First posited in Ancient Greece as the particles into which all matter can be divided, atoms can actually be split into even smaller units, such as electrons. Most visible things are made of atoms, but not all — for instance, light is made of photons.
Mass
A measure of how much gravitational pull something exerts on other objects: gravity is what holds galaxies (and many other things) together. Dark matter also has mass.
Distortions
Even light is affected by gravity, although only very slightly. When light passes by a large mass it bends slightly.
Expanding
Scientists have observed that distant stars are rapidly getting further away from us, meaning that the universe must be getting bigger. What is more, it is expanding at a rapidly accelerating rate. This is strange, because gravity ought to pull the stars closer together.
Foundations
If dark energy does not exist, there is no explanation for the expanding universe. This could mean that Einstein’s theory of relativity — on which much of modern physics is based — is wrong.

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