Dark Matter Teacher Guide - Perimeter Institute

Supplementary Information
Chapter 7
Current Theories of Dark Matter
This chapter of the video
discusses the possibility that there is no dark matter and that, instead,
we need to modify our existing laws of gravity.
describes how most physicists think that dark matter consists of an
as-yet-undetected type of subatomic particle.
Planets
All of the dark matter theories mentioned in Chapter
6 of the video involve objects that have been detected
experimentally. Their failure suggests one of the two
following possibilities:
• dark matter is made of objects that have never been
detected in experiments.
• dark matter does not exist. Much of the evidence for
it comes from effects related to gravity (e.g., the orbital
speeds of stars, gravitational lensing). Thus, if our current
laws of gravity do not apply on a galactic scale, this
evidence is undermined.
WIMPs
Having ruled out all experimentally detected types of
matter as making up the bulk of dark matter, many
physicists turned to undetected varieties. One of the most
popular ideas is that dark matter is made of hypothetical
subatomic particles called “weakly interacting massive
particles” or WIMPs, as in Figure 20.
WIMPs are many times more massive than a proton
and have no electric charge. Electromagnetic radiation
is produced by charged particles, so since WIMPs are not
charged, they do not emit electromagnetic radiation of
any frequency and thus appear dark. Many physicists
are confident that dark matter is made of vast clouds of
WIMPs travelling rapidly in all directions.
Axions
A second theory involving undetected particles is that dark
matter is made of hypothetical subatomic particles called
“axions”. Axions are many times lighter than electrons
34
discusses the two leading candidates for this new particle, weakly
interacting massive particles (WIMPs) and axions.
interviews a number of dark matter researchers on their opinions.
discusses some of the experiments worldwide trying to detect dark
matter directly.
and have no electric charge. One of the main differences
between WIMPs and axions is their mass. Thus, the
difference between the two theories (WIMPs or axions) is
that dark matter is either made of a large number of light
particles (axions) or a smaller number of heavier
particles (WIMPs).
Searching for Dark Matter on Earth
Earth lies within the Milky Way galaxy which is dominated
by dark matter. This means that if dark matter is made of
WIMPs or axions, billions of unseen particles are passing
through your body each second, as in Figure 21. Physicists
should be able to detect a tiny fraction of these particles
(if they exist) using highly sensitive experiments. Thus,
numerous groups worldwide have set up a number of
such experiments, with some of the most promising ones
taking place 2 km underground in a working nickel mine at
SNOLAB in Sudbury, Ontario, Canada.
Figure 20 Many physicists think that dark matter is made of WIMPs.