The largest cohesive objects that we can currently observe in the Universe are galaxy clusters. As the name suggests, these huge structures consist of multiple galaxies (up to several thousand) that are gravitationally bound.
Einstein’s theory of general relativity tells us that, though we consider it be a massless quantity, a beam of light will be bent by a large gravity source. This has been experimentally observed.
When looking at distant objects, it is possible to use astronomical techniques to estimate certain information about that object. For example, using electromagnetic radiation we can determine the chemical nature of a star. There are many ways which we can attempt to determine the mass of a distant galaxy or galaxy cluster, so I will not go into all of them in any great detail. One way includes measuring the doppler shift of the light emitted by different parts of the galaxy to work out how fast they are moving, and calculating mass using orbital mechanics. X-ray emissions can be detected from hot gases inside galaxies, which can again be used to calculate mass. Estimates can also be made using a method called ‘gravitational lensing’, which makes use of the relativistic behaviour of light interacting with gravity. Measurements relating to the ‘apparent’ position of the object (the observed position when the light is bent) and the actual position of the object will give an indication of mass by the magnitude of light curvature.
This is where things get unusual.
Some galaxies have been observed moving at a velocity which disagrees with prediction – a galaxy with that mass would have to be moving slower in order to remain gravitationally bound to the cluster. Furthermore, measurements have been made in which a large portion of measured mass is not accounted for in visible matter.
This has given rise to the notion of ‘Dark Matter’ – a substance that is only detectable indirectly, and cannot be seen.
Dark matter seems to agree with measurement. Simulations which account for it can give the most accurate models of the known universe. It explains why stars at the edge of galaxies orbit faster than they should and why galaxy clusters are so massive. If dark matter is there, it is thought to make up the vast majority of total matter in the universe. Think of it as an invisible web or halo that everything is held within or joined by.
Here’s the thing:
The idea of dark matter is generally mathematically and experimentally consistent, and there is a lot of evidence for its existence. The problem is that many people blindly accept it as scientific fact and consequently never challenge the theory. Remember that no one can actually see dark matter!
I like to use ancient Greek astronomy as an example.
The Greeks were very very clever, especially considering the technology that they had and they developed some very interesting and rigorous models of the solar system. Greek astronomers made various geometric measurements of the moving stars which they named ‘Planets’ (from Planetes, or ‘wanderer’). You may have heard of the Antikythera mechanism, an ancient analogue computer built by the greeks which could predict planetary alignments to an astonishing degree of accuracy. The greeks’ ideas about planets were consistent with their observations, so much so that they were able to build a machine that could make spot-on predictions for them.
Does this mean that the Greeks’ ideas about the solar system were right?
The Greek model says that the entire solar system is held within a giant crystal sphere, upon which the stars are printed. Each planet is part of its own crystal sphere and they are all driven around the Earth by a mechanism contained in the sphere.
You may laugh or claim that the Greeks were just guessing, but we know that these ideas were consistent with observations. This model is based on what they already knew and understood. They understood that to move a rock you had to apply physical force to it, so the planets couldn’t just move on their own! There must be some kind of invisible object there (the crystal sphere and the mechanism which moves them). The Greeks inferred this giant sphere from what they already understood about the world.
We ‘understand’ now that the planets are very different, and that they are set in motion by gravity. We understand that what we touch and feel is made of atoms which are known as ‘matter’. So when we discover that our ideas are inconsistent with observation, we make a theory that is defined in terms of what we understand already (matter and energy). So we infer that there must be some kind of invisible matter that we just can’t see.
Do you notice the similarity?
We mustn’t rule out the possibility that we are wrong and are yet to discover a whole new dimension to the problem. Do you think the Greeks had any inklings about gravity? They may have thought was ridiculous. “Objects cannot simply circle one another in mid-air!” they might argue. That’s because it’s beyond their understanding of the universe.
I would like to stress that I am in no way making claims that dark matter is bad science or that it is wrong (there is very strong evidence for it). I just want to address the problems with accepting it as fact without challenging our understanding.