The best work in science happens when humanity is forced to rethink our understanding of nature.

Aristotle posited that every element (fire, air, earth, and water) had a “natural place”, and when rocks fall or air rises, it is because these elements have a tendency to return to their natural place.1 Based on his observations, Aristotle stated that heavy objects fell faster. This idea was trusted for nearly 2000 years, until scientists observed that objects fall at the same rate despite their mass.

Newton’s laws patched this issue, and instead described falling objects as a result of the force of gravity. Every object attracts each other based on their mass and distance, due to an invisible force called gravity.

Still, Newton’s laws were trusted for about 200 years. Astronomers found that the motion of Mercury orbiting the Sun did not align with Newton’s laws. In 1915, Einstein presented general relativity. This theory described gravity not as a force, but as the geometry of space and time. To date this is one of the most beautiful and accurate theories of the universe, predicting important phenomena like black holes.

This is the cycle of science: equations and laws are formed based on observations, they are tested and trusted until an observation says otherwise, and then scientists reevaluate and get closer to understanding the universe.

One may expect that Einstein’s genius gave the final answer, but astronomers have observed phenomena that general relativity struggles to explain, principally the missing mass problem.

Galaxies are bound systems of stars, planets, gas, and dust which are held together by gravity.2 Earth is bound to the solar system, which is bound to the Milky Way galaxy. According to the current understanding of gravity, galaxies are moving too fast for gravity to hold them together.3 Following their speed and mass, the stars and planets should have been torn away from each other, but alas the Milky Way and trillions of other galaxies survive.

The gravity that holds galaxies together is generated from the total mass of stars, planets, meteors, and other astronomical objects. Newton proved that more mass results in a higher gravitational force. This force holds fast-moving galaxies together, and so there should be enough mass to generate a large enough force. However, an astronomer looking at the sky would find about five times less mass than expected.4

There are two possible explanations: either Einstein’s theory is incorrect, or there is hidden matter that cannot be seen. The latter has been named “dark matter” because it does not emit or absorb light.

Physicists attempting to find a 'correct' version of Einstein’s gravity have been met with dead ends because no single theory can explain all the effects of gravity. However, this does not mean that a new theory can not exist, but that it will be difficult to find. Remember that it took a couple thousand years between Aristotle and Einstein’s theories. However, partially due to the lack of progress in finding a new theory, many scientists believe that the missing piece lies in dark matter.

The simple truth is that this is a blank space in physics. Dark matter has never been detected before, as it is exceptionally difficult finding something that likely does not interact with light or ordinary matter.

Physicists have developed many potential theories. Stephen Hawking proposed that dark matter is actually just black holes that are too small to be observed by modern telescopes.5 Others suggest a cold, slow-moving fluid of particles that provide extra gravity.6 The leading idea is heavy particles that only interact weakly with ordinary matter.

Scientists globally are grappling with the challenge to find a substance whose existence remains unproved. How do you search for something that cannot be seen?

Particle accelerators like CERN and TRIUMF (located at UBC) attempt to create dark matter by colliding particles at high speeds. If dark matter is produced, it would pass through the detector undetected, but there would be a measurable decrease in the overall energy and momentum of the ordinary particles. The missing energy would indicate dark matter was created and left the system.

Additionally, detectors have been launched into space to look for traces of the aftermath of dark matter colliding with itself7. Underground detectors on Earth search for possible interactions of dark matter and ordinary matter.

When it comes to dark matter, we are completely in the dark. Many hours, dollars, and resources have been spent on understanding the missing mass problem. It is difficult to gauge how close we are to an answer, but continuing to study this mystery could unlock a new realm of physics. Does gravity need to be reimagined? Or are hidden particles out there? Newton and Einstein opened new doors with their theories, and exploring dark matter could reveal new physics at the edge of our imagination.