THE NEXT BIG STEPS FOR SCIENCE

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Vera Rubin studied a large number of galaxies and

found that the ef fects of dark matter are widespread

SEARCHING FOR

DARK MATTER

For decades, top astronomers have been on an enormous treasure hunt

for the Universe’s most mysterious substance. But if we can’t see it,

how on Earth do we know it even exists? Colin Stuart explains

Q

Why do scientists think that dark

matter exists?

A

The first clues that everything
in the Universe was not as it
seemed came in the 1930s. Swiss-
American astronomer Fritz Zwicky
was looking at a group of galaxies and
working out how fast the individual
galaxies were moving. To his surprise,
he found them careering around at
speeds far greater than he expected. In
fact, t hey were moving so fast t hat t hey
should have quickly dispersed,
breaking away from the gravity of
ever y t hing else in t he cluster. Except
they weren’t. Zwicky was forced to
surmise that there must be more stuff
in the cluster that was boosting its
overall gravitational pull and keeping
the galaxies tied together. The
discrepancy wasn’t small either. He
estimated there was 400 times more
matter present t ha n he could see. At a
loss to explain what t his mysterious
material was, he called it ‘dunk le
materie’ – German for dark matter.
At the same time, Dutch astronomer
Jan Oort was forced to invoke
something similar. He was looking at
the stars orbiting near the edge of the
Milky Way. He expected to f ind t hat
the further he looked from the galactic
centre, the slower the stars would be
rotating a round it. This idea isn’t
dissimilar to our Solar System: the

further a planet is from the Sun, the

longer it takes to orbit it. But that’s not

what Oort found. The outer stars were

zipping about faster than they should

be. In order to explain why they stayed

bound to the Milky Way despite their

lofty speeds, he supposed there was

some invisible material with

gravitational power spread throughout

the Galaxy. By 1980, American

astronomer Vera Rubin had spotted

t he sa me effect in a round 100 ot her

galaxies. Whatever this invisible stuff

was, it was widespread.

Today, a n effect k now n as

gravitational lensing provides even

more evidence to suggest there is

something strange going on. If we see

a large amount of mass, say a cluster

of galaxies, move in front of a distant

light source, then the foreground

object is able to bend t he light f rom

the background object around it. This

light creates a series of arcs that can

join toget her to for m what’s k now n as

an ‘Einstein ring’. The more mass there

is, the greater the amount of bending.

Yet there is often not enough visible

mass in the cluster to account for the

a mount of bending we obser ve. Again,

there must be extra mass that’s hidden

from view.

Q

What do scientists think dark

matter is?

A

Physicists have a cookbook for

the Universe known as the

Standard Model of particle physics. By

using its recipes, they can account for

the behaviour of forces and the way

particles interact with one another.

This model has been validated many

times over, including by experiments

at CERN’s Large Hadron Collider. The

book’s final missing page was the

recently discovered Higgs boson.

And yet there is nothing within those

recipes t hat allows physicists to cook

up anything with the observed

behaviour of dark matter. It has to be

able to interact wit h nor mal matter via

gravity – and yet in order to remain 5