The Universe, despite all the planets, stars, gases, dust, galaxies and more we find there, doesn’t quite add up. On the largest cosmic scales, we find the same story everywhere we look: there is not enough matter to explain the gravitational effects we observe. Matter sticks together in a cosmic web; galaxy clusters grow to enormous sizes with fast-moving galaxies inside; individual galaxies spin at great speeds that stay great all the way to their edges.
Without the presence of about five times more matter than protons, neutrons and electrons can represent, none of this would be possible. Our picture of the Universe requires dark matter for self-coherence. Yet if dark matter is real, that means our Milky Way also has a dark matter halo, and some of that matter has passed through the solar system, Earth, and even you. Here’s how to know how much is in you right now.
In the young Universe, everything was warmer, denser and more uniform than today. At first, there were regions of very slight overdensity, where there was an above average amount of material. Gravitation preferentially pulls more matter into a region like this, but radiation acts to repel that matter.
If all we had was normal matter and its constituent particles to accompany this radiation, the galaxies and clusters of galaxies that exist today would look very different from what we observe. But if dark matter is present in this 5 to 1 ratio with normal matter, we can theoretically replicate the cosmic web of structure to match our observations and measurements.
A consequence of the existence of dark matter is that it implies that every large structure that forms in the Universe, such as a galaxy, will be surrounded by a large diffuse halo of dark matter. Within the confines of each galaxy, normal (atom-based) matter will accumulate there, as normal matter can collide and interact with both itself and radiation. But dark matter simply passes through everything: itself, normal matter, photons, etc.
Interacting solely by gravity, dark matter particles have no way of losing the great momentum with which they begin. In the entire history of the Universe, each particle of dark matter has passed through the galactic center only a dozen times so far.
At the largest scales, dark matter dominates the Universe. But where we are, just 25,000 light-years from the galactic center, normal matter is locally more abundant than dark matter. Here on Earth, in our solar system, this situation is even more serious than in interstellar space. The density of a human being is comparable to that of water: 1000 kilograms per cubic meter (kg/m3).
Black matter? Even based on the most realistic simulations we can concoct, the local density of dark matter where we find ourselves is many times smaller: around 10-21 kg/m3. If you were to add up all the dark matter inside all humans on Earth at any given time, it wouldn’t add up to more than one nanogram.
If you were to take all the dark matter in the entire solar system, up to the orbit of Neptune, and total it up, that would only be about 1017 kg: the mass of an asteroid of modest size. And yet, because it doesn’t have the collisional interactions that normal matter has, it doesn’t move with the solar system. It’s not :
- orbit around the Sun,
- move with the Sun or other stars around the galactic center,
- stay on a plane,
- or rotate with the Milky Way disk.
In other words, this matter moves under the influence of gravity, relative to the Earth, at quite fast speeds!
If you want to know how much dark matter passes through you in a given amount of time, all you need is four numbers that you can multiply together. They are:
- the density of dark matter,
- the surface of a human that dark matter can touch,
- the speed of dark matter,
- and the time during which you want to know the answer.
Once we have estimated the density of dark matter — and we already have it, at 10-21 kg/m3 – we can get the answer right away.
The surface area of a typical human is 1.7 square meters. Since dark matter comes in at a random angle, we can do a quick calculation and find a good estimate of the area that dark matter “sees” is closer to 0.6m2.
Our solar system orbits the galactic center at speeds of around 200 km/s, but falling dark matter is expected to move relatively faster: closer to 350 km/s. In total, this means that dark matter moves, compared to a human on Earth, at a speed of about 400 km/s.
And we can do it at any time: every second, over the course of a year, or over a typical human lifetime (80 years).
Even if at any given time there are only about 10-22 kilograms of dark matter inside you, much larger amounts are constantly flowing through you.
- Every second you will experience approximately 2.5×10-16 kilograms of dark matter flowing through your body.
- Each year, about 10-8 kilograms of dark matter pass through you.
- And in a human lifetime, a total of just under 1 milligram of dark matter has passed through you.
What may seem like a tiny amount actually adds up over a long enough period of time.
The fact that these numbers are as big as they are tells us not only about our bodies and what’s inside them, but also about how we might dream of searching for dark matter. Whether it’s made up of extraordinarily low-mass or high-mass particles, we know the amount of dark matter mass that passes through not just a human, but any detector in a given volume. If we assume we know the mass of dark matter, we can calculate the number of particles that pass through anything.
For decades, we’ve been building larger, more sensitive detectors, trying to probe the tiny interactions that might exist between dark matter and normal matter. The most advanced detectors today use atoms with large nuclei in extremely large masses, looking for signs of recoil or some other interaction. And so far, all direct detection techniques have remained empty.
Dark matter, as far as we know, is there in all directions. It may be invisible to our eyes, but we can feel its gravitational pull. It passes through all matter in the Universe, including human beings, as if it were not there at all. There are, to our knowledge, no collisions or interactions other than its effects on the curvature of spacetime. It does not clump together, group together or form any structure like dark atoms or molecules.
And yet, if it has the slightest hint of the ability to collide with normal matter or radiation, we will be able to detect it. During your lifetime, approximately one milligram of dark matter will have passed through your body. If even a dark matter particle interacts with a proton or an electron in your body, we’ll have a chance. When it comes to dark matter, one of the deepest mysteries in the universe, it’s hard to ask for more.
Ethan is on vacation. Please enjoy this older article from the Starts With A Bang archives!