The 5 Biggest Cosmic Mysteries That Scientists Are Still Trying to Solve

With all the technological marvels we have today, it is tempting to think that we have all the answers. However, the reality of the situation is far different: many of the fundamental questions about the nature of the universe are still unanswered. Here are the things that keep cosmologists up at night.

Dark Matter: Galaxies Are Weird

We’re all used to the fact that gravity keeps things together—it is gravity that keeps you affixed to the Earth, the Moon orbiting the Earth, and the Earth orbiting the Sun. We understand the interplay between mass and gravity intuitively.

Unfortunately for astronomers, there is something weird about the gravity of galaxies.

Much like how gravity keeps the Earth in orbit around the Sun, gravity also keeps the stars in galaxies orbiting the center of the galaxy. Using a mix of observation and theory, astronomers are able to estimate just how much mass a galaxy contains and can estimate the gravitational forces exerted on stars.

The trouble arises when you start looking at how fast stars orbit within a galaxy. Like the planets in our solar system, you would expect stars on the outer edge of a galaxy to orbit slower than the ones towards the center of the galaxy. However, that isn’t what scientists see. Instead, stars orbit at about the same speed regardless of their distance from the galactic center. You might see this referred to as the “Galaxy Rotation Problem.”

Given the motion of the stars, scientists concluded there must be extra matter that we can’t see distributed throughout galaxies.

Scientists have dubbed this invisible extra matter “dark matter,” and have been hunting for clues to explain it since it was first predicted. No one knows conclusively what dark matter is, but the most popular explanation is that dark matter is made up of weakly-interacting massive (massive relative to protons and neutrons anyway) particles, or WIMPs for short. Unfortunately, by definition, weakly interacting particles are extremely difficult to directly observe.

Bethany Baldwin-Pulcini and Steven Hyatt of UC Davis built a handy web app that lets you explore the relationship between the mass of a galaxy’s black hole, the amount of regular matter, and the amount of dark matter on the speed of stars.

The weird motion of galaxies isn’t the only evidence for dark matter. When light passes through a gravitational field, the path it takes bends, and the bend becomes more extreme as the gravitational field gets stronger. When looking at clusters of galaxies, which have unbelievable mass, you can actually see gravity bending light.

Scientists have calculated how much light should bend based on the amount of matter we can see, and they found that light bends more than expected. The unexpectedly strong gravitational fields are even more evidence that there is a lot of matter in the universe that is invisible to us.

According to NASA, as much as 85% of the universe’s total matter is actually dark matter.

Hubble Tension: We Can’t Agree How Fast the Universe is Expanding

In 1929 the scientist Edwin Hubble discovered something remarkable: the universe is expanding. At the time, it was revolutionary—even Einstein had originally gotten it wrong. In the 1990s, things got even weirder. Not only is the universe expanding, but the rate of expansion is actually increasing, likely due to the mysterious influence of some “dark energy.”

Since then, scientists have been intent on calculating just how fast the universe is expanding using instruments like the Hubble Space Telescope, the James Webb Space Telescope, and Planck.

To do that, they use “standard candles,” which are objects whose luminosity is well understood. That sounds complicated, but it isn’t so exotic as you might think. If you know how much light something is actually giving off, and you know how bright it appears, you can figure out how far away it is. The two most famous examples of these standard candles are Cephied Variable Stars and Type Ia supernovae, both of which have been studied extensively. The picture below marks some Cepheid variable stars in UGC 9391, a galaxy more than 130 million light years from Earth.

On the other hand, you can also examine things like the Cosmic Microwave Background (CMB), which was created only 400,000 years after the Big Bang, to estimate how fast the universe is expanding.

Here’s the trouble when you do the math: you get very different answers. The rate of expansion calculated using “standard candles” is about 9% faster than you get if you measure it using the Cosmic Microwave Background. Scientists are sure why that difference exists, but they’re extremely certain that it isn’t a problem with the measurements. Something else is at work.

A study published in the Monthly Notices of the Royal Astronomical Society suggests that the entire universe is slowly spinning, which could give rise to the discrepancy.

Dark Energy: Why Is the Universe Expanding?

Scientists are confident that the universe is expanding at an increasing rate, but no one is entirely sure how or why it is happening.

The leading explanation is that there is some unknown source of energy, typically called “dark energy,” that is fueling the expansion. Where that energy comes from is also a matter of debate.

The most common theory—usually just called the “Cosmological Constant”— is that space always has a certain minimum amount of energy in it, so as the universe expands, more energy spontaneously appears. Unfortunately, theory can’t currently accurately predict the amount of energy we’ve observed, and the final answer to dark energy will probably require new physics to discover.

Why We Can’t Let the Idea of Warp Drives Go

Make it so.

Matter vs. Antimatter: Why Do We Exist?

Everything you touch in your day-to-day life is made up of matter, sometimes called “baryonic” matter. But there is a problem: it (and you) probably shouldn’t be here.

Much like there are north and south poles on a magnet, and negative and positive terminals on a battery, matter also has an opposite, appropriately named “antimatter” or “anti-baryonic matter.”

Antimatter and matter have equal and opposite properties. For example, when you’re talking about ordinary matter, the nucleus of an atom of made of positively charged protons. With antimatter, the nucleus has a negative charge, because it is composed of negatively charged anti-protons. We can actually produce antimatter in small quantities in labs.

So what happens if you combine matter and antimatter? They annihilate—violently. Matter-antimatter reactions are 100% efficient. The combined mass of both the matter and antimatter are converted into energy.

If matter and antimatter had been created in equal amounts at the beginning of the universe, none of us would be here today—the matter and antimatter would have annihilated each other. Scientists have been surveying objects in deep space, like the Bullet Cluster below, in an attempt to find signs of antimatter from the beginning of the universe, with little success. The fact that we’re here, and that so much of the observable universe is made of matter, is weird.

That gets to the heart of the mystery: why is there so much matter and so little anti-matter? So far, no one knows.

The Big Bang

The Big Bang is generally supported by the evidence scientists have collected over the last century, but there still isn’t a good answer for “Why did it go bang in the first place?”

One explanation is that the Big Bang was caused by a quantum fluctuation. Quantum physics predicts that space isn’t actually empty. Rather, particles are spontaneously popping into existence before quickly being annihilated all over the place, all the time. It may be that the universe is the result of one particularly spectacular fluctuation.

There are other, even more exotic theories, too. One such theory, called “Brane Cosmology,” comes from String Theory. The idea is that the universe is composed of large structures called branes that exist in up to 11 dimensions. If they exist, it is thought that these branes will interact with each other, and sometimes even collide. Such collisions could create a universe like our own.

Interestingly, this explanation also allows for the existence of a multiverse, where more than one universe exists.

Unfortunately, like most predictions made by String Theory, we can’t currently test for the existence of branes, and it is entirely possible that a concrete answer will always be out of reach.

It may seem that all of these mysteries can be resolved given enough time or scientific ingenuity, but we shouldn’t take that for granted. Many of these questions are tied up in events that happened billions of years ago, in conditions so extreme that we wouldn’t even recognize them as our own universe. Some of these ideas, especially those that hinge on string theory, may permanently be beyond our reach. At a minimum, we’re guaranteed to need exciting new physics to come up with a satisfying answer.