The vast majority of mass in our universe is invisible. And for quite some time, physicists have been trying to understand what this elusive mass is. If it is made up of particles, the hope is that the Large Hadron Collider can produce a dark matter particle, or the space telescope will see the eloquent gamma ray signature of a dark matter collision. So far, there is nothing. And this problem makes theoretical physicists ponder new ideas.
In 2017, renowned theoretical physicist Lisa Randall took a peek into one of the most incredible possibilities of dark matter. Hypothetical, of course. Rather than treating dark matter as a particular type of particle, she assumed that dark matter could be made up of a whole family of particles that make up dark stars, dark galaxies, dark planets, and possibly dark life. The chemistry of the dark universe could be as rich and varied as our own "regular chemistry."
But it's not that simple.
Dark matter problem
Our Universe is an amazing, albeit incomprehensible place.
Over the past few decades, we have come to realize that 84.5% of the matter in the Universe cannot be seen. Given its rather awkward nickname "dark matter", this substance is in a state in which it does not interact with "normal" matter. Like dark energy, these things are “dark” because we don't understand them.
If there is a piece of dark matter on my desk now, I will never know about it. A piece of dark matter in general, as such, cannot lie on my desk. It will fall through the table, and the floor, and the earth's crust, rush into the gravity well at the core of our planet. Or it will disappear into space in an incomprehensible way. Dark matter interacts so weakly with anything that this piece will simply fall through ordinary matter as if it does not exist.
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On a small scale, the gravitational manifestation of dark matter is negligible, but at cosmological distances, the presence of dark matter is definitely felt - it can be observed indirectly by its gravitational effect on galaxy clusters and its effect on the rotation of galaxies. We know that it exists, we just don't see it.
And we don't know what it is. We can only guess.
Ordinary matter - aka baryonic matter - interacts through electromagnetic, gravitational, strong and weak forces. These forces transfer energy and give structure to all matter. Dark matter, on the other hand, is usually viewed as an amorphous cloud of "matter" that cannot interact through electromagnetic, weak or strong forces. Therefore, dark matter is assumed to be "non-baryonic". Non-baryonic matter can reveal its presence only gravitationally.
The leading candidate in the search for dark matter is WIMP, a weakly interacting massive particle. As the name of the WIMP suggests, this hypothetical particle does not interact with normal matter - so it is not baryonic.
Established cosmological models predict that dark matter - be it in the form of WIMPs or "axions", say - endows our Universe with structure and is usually simplistically called the "glue" that holds our Universe as a whole.
Observing the rotation of galaxies, astronomer Vera Rubin noticed that most of the matter in galaxies is not observable. Only a small percentage are visible - stars, gas and dust; the rest hides in a huge but invisible halo of dark matter. It's like our visible galaxy of ordinary matter is just a hood on a huge wheel of dark matter that extends far beyond what we can see.
In a recently published paper (2013), Randall and her colleagues presented a more complex form of dark matter. According to them, the dark matter halo of our galaxy does not consist of only one type of amorphous mass of non-baryonic matter.
“It seems very strange to assume that all dark matter is composed of just one type of particle,” writes Randall. "An unbiased scientist should not allow dark matter to be as diverse as our normal matter."
A rich "shadow universe"?
Just as our visible universe is governed by the Standard Model of physics - a well-tested family of particles (including the infamous Higgs boson) and forces, could a rich and varied model of dark matter particles and forces function in a dark galactic halo?
This research follows the logic of assuming a rich variety of unknown physics in the dark sector of the universe - let's call it the "shadow universe" - that runs parallel to our own and has all the complexities our visible universe has to offer.
Astrophysicists have previously suggested that "dark stars" - stars made of dark matter - may exist in our ancient universe to this day. If so, Randall argues, perhaps "dark planets" could form. And if there is a family of dark matter particles controlled by forces deployed in the dark sector, could this lead to complex chemistry? And to life?
However, if “dark” or “shadow” life exists parallel to our universe, you can forget that we can detect it.
Shadow life will remain in the shadows
It seems tempting to use this hypothesis to explain all the day-to-day mysteries, or even paranormal claims, that science cannot dispute or support. What if “ghosts” or inexplicable “lights in the sky” are the antics of dark creatures living in the back of everything?
While this logic would be fine for a TV show or movie, these dark creatures would live in a shadowy universe that is completely incompatible with ordinary matter. Their particles and forces would have no effect in our universe. You could read these lines sitting on a tree stump in a dark forest, and you would never know about it.
But since we coexist with this shadow universe in the same space-time - no extra dimensions or multiverse - only one signal can be transmitted.
Gravitational waves were only discovered in 2016, and the first detection of these ripples in spacetime was caused by the collision of black holes. It seems quite possible that gravitational waves can be detected in the dark sector, but only the most powerful cosmic events in the dark sector can be detected at our end of the wire.
All in all, we'll almost certainly never prove the existence of cute dark matter creatures, but Randall makes a point. When we contemplate the source of dark matter, we must look beyond our prejudices; the dark sector can be a complex family of dark matter particles and forces that are beyond what we can imagine.
Ilya Khel
