What if the universe we know of is only a fraction of a larger, mostly invisible universe and we can only interact through gravity?
This is just one of the ideas that physicists pursue in their quest to solve the mystery of dark matter.
I love the idea of dark matter. For me it's the ultimate puzzle. I mean, our universe contains protons and neutrons and electrons and that's great. All of this "normal" matter is the basic ingredient for everything from Brad Pitt to intergalactic gas clouds. Normal matter also exerts a gravitational force, and that's great too. Gravity keeps my coffee from floating and prevents galaxies from drifting apart.
But adding all the particles in all the galaxies in the universe does not suffice to create all the gravity in the universe. There is much too much gravitational force and too little matter.
In order for our Universe to make sense, it takes a whole load of something extra to make up for the lack of gravity.
Something extra turns out to be 80% of matter in our universe. We call it "dark" matter: particles we can not see and do not understand, but which bring enough gravitational force to balance the books.
The Hunt for Missing Matter
We have been looking for direct evidence of dark matter for years, but found nothing. Do not worry, we knew it would be difficult. Researchers believe that dark matter virtually ignores normal matter and light.
"Dark matter particles could be all around us, but they just do not interact electromagnetically," says Vid Iršič, an expert at the University of Washington. "They could only interact gravitationally."
Watch out! Passing Neutrinos
Although it sounds strange, there is already a precedent for particles that do not play well with others. They are called neutrinos.
"Neutrinos are very, very light particles that interact very weakly," says Vid. "They come from the stars, neutrinos from our sun cross the earth billions of heights every second, every square centimeter."
These particles are just penetrating you, but their interactions are so mild that you feel nothing. Nevertheless, neutrinos are like big brass bands compared to dark matter.
So what are dark matter particles?
There are a lot of theories and experiments that are looking for many different particles in the race to solve the mystery of dark matter. In the following, we describe Vid's personal top five candidates for dark matter, from the most likely to the most unlikely. Thinking of Caps
Candidate # 1
: Weakly Interacting Massive Particles (or WIMPs)
Weakly interacting massive particles are a collective term for a group of particles that fit this description. "Weakly interacting" means that they do not interact much with normal matter or light. "Massive" means that they are larger than the mass of a proton.
WIMPs are popular because they were independently predicted by several different particle physics theories. And because of the WIMP miracle. The wonder is that these theories also predict the total mass of WIMPs, and it's about the same mass needed to explain the extra gravitational force. Coincidence? Maybe not.
Candidate # 2: Axions
Axions are a theoretical particle that we have come up with to solve an unsolved problem of quantum dynamics. More specifically, their existence would help explain quirks in our understanding of quarks and how they stay together to form protons and neutrons. But that's a very different story …
If axions exist, they would also tick some boxes of dark matter. It is predicted that they interact only weakly with ordinary matter and light. And they would be just the right size for us to have (until now) missed. (This size, if you wonder, is somewhere between 2 and 100 micro-electron volts (and a micro-electron volt is 1.78 × 10 -42  kilograms).
Axions are also super-baffling: a Axion breaks down into two photons and one can create an axion by combining two photons, which hopefully will help us recognize them in the near future. In April, Vids University announced that it can now seek more sensitively than
Candidate # 3: Ultralight scalar dark matter
This candidate is a relatively new kid on the block, also known as ultralight axions, fuzzy dark matter or wave dark matter  If it exists, it's super-duple light, about 10 – 22 electron volt, and because it is so light, it behaves in a way that is consistent with string theory and quantum mechanics.The idea is, when you bring together a whole galaxy charge of these particles, they behave like me hr like a wave as a spinning sphere
They end up with tiny particles that fuse into a wave that spans thousands of light-years. No wonder they are difficult to find.
Candidate # 4: Sterile Neutrinos
Dark matter could also be a special form of neutrino, which is difficult because neutrinos are already something special. They come in different flavors (electron, muon and dew) and they can switch between flavors as they travel across the room. Moreover, they are minimally interactive and super light.
Neutrinos of Dark Matter, if they exist, are the much heavier cousins of the neutrino. Called sterile neutrinos, it is predicted that they interact with normal matter only when they turn around between flavors.
This is an increasingly popular option. What if there is not just one type of dark matter particle, but many?
Just as normal matter has a lot of different particles, so could dark matter. But because normal particles and dark particles do not interact much, we never know. Perhaps we could observe these particles only indirectly through their gravitational effect on the evolution of the cosmos. The thoughts explode.
"That's pretty intriguing," Vid says, which could be the biggest understatement in the world.
"More and more people believe that dark matter is not just a particle, but a collection of particles, perhaps in a mirror image of the world we know, the standard particles we know but separate from gravity and interact with us in no other way. "
Will we ever find out what Dark Matter really is? Well, we do our best. Thousands of scientists around the world are searching for signs, testing new ideas and developing breakthrough technologies.
We always thought the world was flat. And that our universe was visible. What will the next decades bring?
This article was originally published on Particle. Read the original article.