The vast majority of mass in our universe is invisible, and for quite some time, physicists have been trying to understand this elusive mass. If it is composed of particles, there is hope that the Large Hadron Collider can produce a dark matter particle or that the space telescope will see the telltale gamma-ray signature of dark matter particle collisions. So far, there is nothing, and this problem forces theoretical physicists to reflect on new ideas.
In 2017, renowned theoretical physicist Lisa Randall looked into one of the most incredible possibilities of dark matter. Hypothetical, of course, instead of treating the dark matter as a specific type of particle, she conceded 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 “ordinary chemistry”. But not everything is so simple.
The Problem of Dark Matter
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 clumsy nickname of “dark matter,” this matter is in a state where it doesn’t interact with “ordinary” matter. These things are “dark” like dark energy, because we don’t understand them.
I will never know if there is a piece of dark matter on my desk right now. A piece of dark matter in general, as such, cannot lie on my table. It will fall through the table, the floor, and the earth’s crust, rush into the gravity well in the core of our planet. Or he will escape into space incomprehensible. Dark matter interacts so weakly with anything that this piece will simply fall through ordinary matter, as if it were not there.
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 through its gravitational effect on galaxy clusters and its influence on the rotation of galaxies. We know 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 transmit energy and give structure to all matter. On the other hand, dark matter is usually viewed as an amorphous cloud of “stuff” 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,” for example—endows our universe with structure and is commonly referred to in a simplistic way as the “glue” that holds our universe together.
Observing the rotation of galaxies, astronomer Vera Rubin noticed that most of the matter in galaxies is not observable. Only a small percentage is visible – stars, gas, and dust; the rest is hidden in a huge but invisible halo of dark matter. It’s as if our visible galaxy of ordinary matter is just a cap on a huge wheel of dark matter that stretches far beyond what we can see.
In a paper published (2013) by Randall and her colleagues, a more complex view of dark matter was presented. According to them, the dark matter halo of our galaxy does not consist of just one type of amorphous mass of non-baryonic matter.
“It seems very strange to assume that all dark matter is made up of just one type of particle,” Randall writes. “An open-minded scientist should not assume that dark matter will not be as diverse as our ordinary matter.”
A rich “shadow universe”?
Just as the Standard Model of physics governs our visible universe—a well-tested family of particles (including the notorious Higgs boson) and forces—can a rich and varied model of dark matter particles and forces function in a dark galactic halo?
This study follows the logic of suggesting a wide variety of unknown physics in the dark sector of the universe – let’s call it the “shadow universe” – which exists parallel to our own and has all the complexities that our visible universe has to offer.
Astrophysicists have previously speculated that “dark stars” — stars made of dark matter — could exist in our ancient universe to this day. If so, Randall speculates, perhaps “dark planets” could also form. And if there is a family of dark matter particles governed 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 Remains in the Shadows
It seems tempting to use this hypothesis to explain all everyday mysteries or even paranormal claims that science cannot dispute or confirm. What if “ghosts” or inexplicable “lights in the sky” are the antics of dark creatures living in the underside of everything?
While such logic would be fine for a TV series or movie, these dark creatures would live in a shadow universe that is completely incompatible with ordinary matter. Their particles and forces would have no influence in our universe. You could read these lines, while sitting on a 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 multiverses – only one signal can be transmitted.
Gravitational waves were only discovered in 2016, and the first detection of this ripple in space-time was caused by black holes colliding. It seems quite possible that gravitational waves could be detected in the dark sector as well, but only the most powerful cosmic events in the dark sector can be detected at our end of the wire.
All in all, we will almost certainly never prove the existence of cute dark matter creatures, but Randall makes an important point. When we think about the source of dark matter, we must go beyond our prejudices; the dark sector may be a complex family of dark matter particles and forces that are beyond what we can imagine.