The universe's strangest elementary particles: physicists remain uncertain about their existence.

All matter in the universe is made up of fundamental particles such as quarks, protons, neutrons, electrons, and neutrinos. Each of these particles has its own peculiar characteristics. However, there are also hypothetical particles that are so strange that physicists are not even sure they exist at all. Here are five of the most bizarre and rare hypothetical elementary particles in the universe, as reported by Space.

Dark Photon

Most people are familiar with the ordinary photon – the particle of light. However, physicists believe there is also a dark photon, which is similar to the ordinary photon but simply dark. Dark photons are thought to be carriers of an unknown fundamental interaction and may be components of dark energy and dark matter.

Dark matter is an invisible form of matter that makes up a significant portion of the mass of nearly every galaxy. Dark energy is responsible for the accelerated expansion of the universe. Together, dark energy and dark matter are believed to constitute a large part of the energy and matter in the universe.

Physicists are not exactly sure if the so-called dark side of the universe is as complex as its light side, which consists of ordinary matter and energy. If the dark side is equally complex, there could be additional fundamental forces in the universe that act only between dark matter and/or dark energy. Dark photons could be the carriers of these unknown forces.

So far, scientists have not found evidence of the existence of dark photons, and they remain merely hypothetical particles.

Curvaton

Physicists believe that in the first moments after the Big Bang, the universe underwent a period of incredibly rapid expansion known as cosmic inflation. This expansion is thought to have been driven by a mysterious particle known as the inflaton, an extremely strong version of dark energy.

It is believed that cosmic inflation laid the groundwork for the formation of stars, galaxies, and galaxy clusters that appeared much later.

Physicists do not fully understand cosmic inflation and how it set the stage for the entire structure of the cosmos. Therefore, scientists have suggested that alongside the inflaton, there exists a hypothetical particle called the curvaton.

It is proposed that only after the end of cosmic inflation did the curvaton help create the structure of the universe as we know it. The downside of this hypothesis is that instead of one hypothetical particle, the existence of two is suggested to explain cosmic inflation. This particle has also not been detected, but searches are ongoing.

Gluonball

One of the carriers of the four fundamental forces of the universe, specifically the strong interaction, is a particle called the gluon, which exists in nine varieties.

Gluons are responsible for the strong interaction between quarks, which make up protons and neutrons. The latter are baryons consisting of three quarks and gluons, while a group of particles known as mesons contains two quarks and a gluon.

However, physicists believe there exists a hypothetical particle called the gluonball, which is made up solely of gluons held together by the strong interaction.

What makes the gluonball elusive is that it may exist for less than a microsecond. The Standard Model of particle physics predicts that gluonballs surprisingly exist and can be detected using particle colliders.

X17

The existence of a hypothetical particle called X17 was announced in 2015 by Hungarian physicists. The scientists created a setup to search for dark photons. In this setup, protons were directed at a lithium-7 target, which then transformed into beryllium-8 nuclei that subsequently decayed, producing pairs of electrons and positrons. If physicists detected more of these particles than expected, it could indicate the presence of dark photons.

Physicists observed an anomalous increase in the number of electrons and positrons and suggested that this may be due to an unknown particle with a mass of 17 MeV, which is 34 times the mass of an electron. Hence, the mysterious new particle was named X17.

Although further studies increasingly pointed to the existence of such a particle, there is currently no convincing evidence for it.

Preon

It is important to remember that atoms consist of protons, neutrons, and electrons. The first two particles, in turn, are composed of quarks. However, physicists believe that quarks and leptons (which include electrons, muons, and neutrinos) are made up of a hypothetical particle known as the preon.

One of the main reasons for the existence of preons is that many particles are extremely similar to each other, differing only in minor ways. Currently, physicists have no explanation for these nearly identical properties, leading to the hypothesis that they may arise from some unknown interactions.

Physicists have proposed preons to explain nearly every unresolved issue in the Standard Model of particle physics. However, no evidence has yet been found to confirm that quarks and leptons are composite particles.