What if I told you that trillions of particles are passing through your body right this moment? You can’t see, hear, or touch them, yet they exist in such a vast quantity that they outnumber all the visible matter in the universe by six times. This hidden matter overshadows stars, galaxies, planets, gases, and even radiation. Remarkably, despite its dominance, we can’t detect these particles with any instrument. So, how do we even know they exist?
To answer that, let’s rewind to the late 1970s when Vera Rubin, a trailblazing astronomer, was making waves in a field dominated by men. She was delving into a largely ignored area of astronomy, studying the movement of stars in the Andromeda galaxy. What she discovered was nothing short of groundbreaking. Rubin found that stars on the outer edges of the galaxy moved at nearly the same speed as those closer to the center. This observation defied Newton’s law of gravitation, which had held steady for 300 years.
Newton’s laws dictate that the further an object is from its center of mass, the lower the gravitational force acting on it, hence it should move slower. This principle is plainly observable in our solar system, where planets far from the Sun orbit more slowly. If our own solar system behaved like the stars in Andromeda did, it would fall apart.
Rubin’s findings showed that something wasn’t right. Subsequent studies revealed that this peculiar behavior was not limited to Andromeda but was common in all galaxies, including our own Milky Way. This led to a striking conclusion: there must be much more mass in the galaxies than we can observe directly. However, this mass doesn’t emit light or radiation and doesn’t interact with ordinary matter, except through its gravitational pull. It became known as dark matter.
Dark matter is aptly named because it is invisible to all our senses and instruments, yet it has at least six times the gravitational pull of all observable and detectable matter in the universe. To put it in perspective, dark matter comprises about 85% of all matter, leaving us effectively blind to the majority of the universe.
Even more astonishing, when you combine dark matter with dark energy—a topic for another day—it turns out that we can only see about 4% of the universe. The remaining 96% consists of dark energy and dark matter, unseen and undetectable. So, in essence, our observable universe is just the tip of the iceberg, the foam atop the vast ocean of reality.