It can safely be presumed that the vastness of the Universe has left most of us with an existential crisis of sorts at least once in our lifetimes. Putting aside the Universe as a whole for a minute, if we were to look at just the observable Universe, a region that spans across 46 billion light-years, we might find at the very least 2 trillion galaxies notwithstanding the various other astronomical objects. Yet, if we were to pile all this matter up we’d be left with a measly percentage – only 5% of the observable Universe is composed of matter. The rest? It’s a whole bunch of theories that can be summed up by two terms – dark matter and dark energy. It is estimated that roughly 68% of the Universe is composed of dark energy while dark matter makes up the remaining 27%.
Dark Matter and Baryonic Matter
We can estimate that 80% of the Universe’s mass is made up of dark matter. There’s not a lot that’s been unravelled about this obscure material beyond this. However, scientists have come up with a checklist that tells us what dark matter is probably not.
For one, dark matter isn’t matter. It’s an easy conclusion to draw given that there likely aren’t enough visible entities in the Universe, be it stars or planets, beyond the scope of normal matter. If we can lay our eyes on something, chances are it’s normal matter, be it the stuff we run into in our day-to-day lives or celestial bodies. Matter is composed of certain subatomic particles called baryons (of which protons and neutrons are examples of), which are, generally speaking, composite particles made up of three quarks (quarks are tinier subatomic particles). As a result of this, it’s often referred to as baryonic matter. Dark matter, on the other hand, is speculated to be non-baryonic in nature and contains subatomic particles that are yet to be discovered. These particles are responsible for dark matter being virtually undetectable – they don’t interact with electromagnetic radiation (such as light) by means of absorption, reflection or emission. That’s another thing we know about dark matter: it’s invisible (for the most part).
Dark Matter and Antimatter
On the flip side, dark matter isn’t antimatter either. But first, a brief overview: subatomic particles, in general, have three intrinsic properties to them – mass, charge and spin. Antiparticles (the particles of antimatter) are exactly like particles in that they have an identical mass and spin. However, the trait that differentiates antimatter from matter is that the two of them are oppositely charged, and when they meet, they result in an explosion and a slew of gamma rays.
These gamma rays clue us in on the whereabouts of antimatter in the Universe, which is pretty scarce in actuality but that’s something we’ve previously touched upon here. Having said that, when it comes to dark matter and its interaction with matter, no surge in gamma rays is detected
Dark Matter’s Conception
Speculation for the existence of dark matter came about due to its gravitational effects on baryonic matter. For instance, it was inferred that the stellar rotational velocity in spiral galaxies would be greater at the galactic centre than at the outer edge. The decrease in velocity as the stars moved away from the centre fell in line with the laws of physics. However, upon experimentation, it was discovered that the velocity remained constant across the span of the galaxies, implying that they were far more massive than previously thought. Herein enters dark matter and its mass, forming a halo around the galaxy, contributing to its mass and gravitational pull on the stars.
If that’s not convincing enough, another reasoning revolves around galaxy clusters. The speed with which clusters of galaxies travel at ought to break them apart, not to mention the damage bestowed from the searing heat of gas clouds. Dark matter comes to the rescue yet again with the notion that its gravity is the glue that holds the clusters and hot gas clouds together.
As for locating dark matter in spite of its ‘invisibility’, a phenomenon called gravitational lensing is used. A gravitational lens occurs when a large bunch of matter, say a cluster of galaxies, results in the bending of light rays from distant galaxies due to the cluster’s gravitational field. By studying the distortions thus produced, a map of dark matter in the Universe is being created.
Dark Matter and its Unlikely Candidates
The term, dark matter, by itself, functions as a placeholder as we are yet to stumble upon what makes it tick. If dark matter was to be baryonic in nature (which is a big if) it would have to be in the form of MACHOs or Massive Compact Halo Objects. Since we detect baryonic matter by means of its interaction with electromagnetic radiation, the working theory is that such matter must emit little to no radiation in order to be a contender. Such objects do in fact exist in the form of MACHOs, thus making them ideal candidates for our dark matter dilemma. Examples of MACHOs range from black holes and neutron stars to brown, white and very faint red dwarfs. Nevertheless, the aforementioned predicament with dark matter being a form of baryonic matter still holds true – there just isn’t enough.
Instead, our next best bet is a class of non-baryonic particles called WIMPs or Weakly Interacting Massive Particles. Let me digress to neutrinos: they are subatomic particles with absolutely no electric charge and a minute mass, weighing about 500,000 times lesser than an electron. They’re pretty impressive considering that they’re the lightest subatomic particles (with mass) thus far, and also one of the most abundant ones in the Universe, though their limited interaction with baryonic matter makes it very difficult to detect them. Neutrinos’ limited interaction with normal matter would make them ideal candidates for dark matter, if not for the fact that they weigh next to nothing. That’s not a great trait for a substance that accounts for around 85% of the Universe’s mass. So scientists have hypothesised the existence of WIMPs in the form of particles called neutralinos, which are heavier and slower than neutrinos and far more ideal.
Dark energy is far more obscure than dark matter, in comparison. We’re still not sure what it really is. It’s geared towards the accelerated expansion of the Universe, and our inability to understand why it accelerates in the first place. We were of the notion that the expansion of the Universe would slow down over time, given the attractive nature of gravity. However, upon further testing, the latter was found to be true, and this proved to be funky. Dark energy was the solution to this dilemma.
Dark energy fits into a version of Einstein’s general relativity equation adapted for a static universe, consistent with the cosmological constant. The constant would function as a repulsive force working to counter gravity, something akin to a seesaw being strategically balanced. It essentially boils down to empty space possessing energy, and an expansion of space being accompanied by a proportional amount of energy. While this does explain the accelerated expansion, as the energy overcomes gravity’s pull, the reason for its existence is still a mystery in itself.
Dark energy isn’t limited to just one theory. Another possibility is derived from the quantum theory, wherein, empty space is composed of certain particles that appear and disappear continually. However, calculations haven’t been able to simulate a reasonable number for the amount of energy this might create.
The Universe is Stranger Than Fiction
Dark matter and dark energy are both placeholder terms, biding away the time, as scientists hack away at the puzzle that is the Universe, while simultaneously forming both the clues as well as the puzzle itself. It’s both perplexing and fascinating to put into perspective that 95% of the contents of the Universe remain a mystery. The search for dark matter and dark energy is often compared to our search for the Aether in the yesteryear, a hypothetical medium through which light would travel through, which is a theory that is now defunct. However, the best trait of the scientific method is the ability to hypothesise and theorise until something sticks, until it doesn’t any longer. For now, they’re well accepted theories that might help us get closer to understanding the Universe a little bit better. As for the future, you never know. Billions of neutrinos pass through our bodies every second; maybe our mystery dark matter particles are doing the same.
Written by Shweta Manoharan