- According to the theory of dark matter, it's made of particles distributed all throughout space, clustered in a halo surrounding almost every galaxy.
- In the Milky Way, that dark matter should be everywhere, including passing through our own bodies on a continuous basis, just like neutrinos.
- Over the course of a human lifetime, the total amount of dark matter to pass through your body doesn't even add up to one gram. Maybe that's why it's so hard to detect?
The Universe, despite all the planets, stars, gas, dust, galaxies, and more we find within it, doesn’t quite add up. On the largest cosmic scales, we find the same story everywhere we look: there isn’t enough matter to account for the gravitational effects we observe. Matter clumps into a cosmic web; galaxy clusters grow to enormous sizes with fast-moving galaxies inside; individual galaxies rotate at large speeds that remain large all the way to their edges.
Without the presence of about five times as much matter as protons, neutrons, and electrons can account for, none of this would be possible. Our picture of the Universe requires dark matter for self-consistency. Yet, if dark matter is real, that means our Milky Way has a dark matter halo too, and some of that matter passed through the Solar System, Earth, and even you. Here’s how to know how much is inside you right now.
Back in the young Universe, everything was hotter, denser, and more uniform than it is today. Early on, there were regions of ever-so-slight overdensity, where there was a greater-than-average amount of matter there. Gravitation preferentially attracts more matter to a region like this, but radiation works to push that matter back out.
If all we had was normal matter and its constituent particles to go with this radiation, the galaxies and galaxy clusters that exist today would be vastly different than what we observe. But if dark matter is present in this 5-to-1 ratio with normal matter, we can theoretically reproduce the cosmic web of structure to match our observations and measurements.
One consequence of dark matter’s existence is that it implies that every large structure that forms in the Universe, such as a galaxy, will have a large, diffuse halo of dark matter surrounding it. In the inner reaches of each galaxy, normal (atom-based) matter will collect there, since normal matter can collide and interact with both itself and with radiation. But dark matter simply passes through everything: itself, normal matter, photons, etc.
Only interacting gravitationally, dark matter particles have no way to lose the large momentum they start off with. Throughout the whole history of the Universe, each dark matter particle might plunge through the galactic center only a dozen times by the present day.
On 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, that situation is even more severe than in interstellar space. The density of a human being is comparable to that of water: 1000 kilograms per cubic meter (kg/m3).
Dark matter? Even based on the most realistic simulations we can concoct, the local density of dark matter where we are is many times smaller: about 10-21 kg/m3. If you were to add up all the dark matter inside all the humans on Earth at any one instant, it would come out to no more than a single nanogram.
If you were to take all the dark matter in the entire Solar System, out to the orbit of Neptune, and total it up, it would only add up to about 1017 kg: the mass of a modestly large asteroid. And yet, because it doesn’t have the collisional interactions that normal matter does, it doesn’t move with the Solar System. It doesn’t:
- orbit the Sun,
- move with the Sun or the other stars around the galactic center,
- stay in a plane,
- or revolve with the Milky Way’s disk.
In other words, this matter moves under the influence of gravity, relative to Earth, at pretty rapid speeds!
If you want to know how much dark matter passes through you in a given amount of time, all you need are four numbers that you can multiply together. They are:
- the density of dark matter,
- the surface area of a human being that the dark matter can hit,
- the speed of the dark matter,
- and the amount of time you want to know the answer for.
Once we’ve estimated the dark matter density — 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 the dark matter comes in at a random angle, we can do a quick calculation and find a good estimate for the area the dark matter “sees” is more like 0.6 m2.
Our Solar System orbits the galactic center at speeds of around 200 km/s, but infalling dark matter should be moving relatively quicker: closer to 350 km/s. All told, that means dark matter moves, relative to a human on Earth, at a speed of around 400 km/s.
And we can do this for whatever length of time we want: every second, over the course of a year, or over a typical (80 year) human lifetime.
Even though, at any given instant, there’s only around 10-22 kilograms of dark matter inside you, much larger amounts are constantly passing through you.
- Every second, you’ll experience about 2.5 × 10-16 kilograms of dark matter passing through your body.
- Every year, approximately 10-8 kilograms of dark matter move through you.
- And over the course of a human lifetime, a total of just under 1 milligram of dark matter has passed through you.
What might seem like a minuscule amount really does add up over a long enough period of time.
The fact that these numbers are as large as they are not only teaches us something about our bodies and what’s in them, but how we might dream of searching for dark matter. Whether it’s made 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 of a given volume. If we presume to know the mass of dark matter, we can calculate the number of particles that go through anything.
For decades, now, we’ve been building larger and more sensitive detectors, attempting to probe whatever minuscule interactions 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 a recoil or other interaction. And so far, all the direct detection techniques have come up empty.
Dark matter, to the best of our knowledge, is out there in all directions. It may be invisible to our eyes, but we can feel its gravitational force. It passes through all the matter in the Universe, including human beings, as though it weren’t there at all. There are, to the best of our knowledge, no collisions or interactions other than its effects on curving spacetime. It doesn’t clump, cluster, or form structure like dark atoms or molecules.
And yet, if it has even the tiniest hint of an ability to collide with either normal matter or radiation, we’ll be able to detect it. Over the course of your life, about a milligram of dark matter will have passed through your body. If even one dark matter particle interacts with one proton or electron in your body, we’ll have a chance. When it comes to dark matter — one of the Universe’s deepest mysteries — it’s hard to ask for anything more.
Ethan is on vacation. Please enjoy this older article from the Starts With A Bang archives!