From: veritasium
Dark matter is hypothesized to constitute approximately 85% of all matter in existence, potentially forming a “shadow universe” five times more massive than everything visible to us [00:00:16] [00:01:20]. The existence of dark matter is proposed to explain various astronomical observations that cannot be accounted for by ordinary, visible matter alone.
Early Origins of the Hypothesis
The concept of unseen matter was first introduced in 1933 by Swiss astronomer Fritz Zwicky [00:03:43]. While studying the Coma cluster, a collection of over a thousand gravitationally bound galaxies, Zwicky observed that these galaxies were orbiting their collective center of mass at speeds much faster than expected [00:03:45] [00:03:57]. He concluded that there must be significantly more mass in the cluster than what could be seen, pulling everything inwards [00:04:04]. To explain this, Zwicky proposed the existence of “dunkle materie” (dark matter), from which the modern term originates [00:04:11] [00:04:17]. His idea, however, was not widely accepted at the time [00:04:20].
Evidence from Galactic Rotation Curves
Around 40 years after Zwicky’s initial proposal, more evidence for dark matter emerged [00:04:22]. Vera Rubin and Kent Ford observed the motion of stars in the Andromeda Galaxy [00:04:26]. Standard gravitational theories predicted that stars further from the galactic center would orbit slower [00:04:31]. However, Rubin and Ford found that the rotational velocity of stars remained almost constant with increasing distance from the center [00:04:39]. Without additional unseen mass, these outer stars should have been flung into space [00:04:45].
This consistent rotational velocity was also observed in other galaxies [00:04:51]. Using radio telescopes, Albert Bosma and others measured hydrogen gas even further out from galactic centers and found the rotational velocity still remained constant [00:04:53]. The simplest explanation for these observations is the presence of invisible matter, or dark matter, holding galaxies together [00:05:05]. By analyzing these rotation speeds, scientists estimate that approximately 85% of a galaxy’s mass is composed of dark matter [00:06:10].
Alternative Theories vs. Dark Matter
While the existence of dark matter is the leading explanation, some propose modifying our theory of gravity instead [00:06:19] [00:06:23]. This approach suggests that the unusual centripetal acceleration seen in galactic outskirts is not due to an additional force from dark matter, but rather a fundamental limit to how low acceleration can go (e.g., Modified Newtonian Dynamics, or MOND) [00:07:00] [00:07:07]. However, the scientific consensus strongly favors the idea of dark matter as a physical substance, implying the existence of unseen particles [00:07:16].
Further Evidence
The Bullet Cluster
The Bullet Cluster provides compelling evidence for dark matter [00:07:29]. This is a site where two galaxy clusters have collided [00:07:31]. During the collision, most of the ordinary mass, which is primarily interstellar gas, interacted, heated up, and slowed down, accumulating in the middle [00:07:36]. However, by using gravitational lensing – the way gravity bends light – scientists measured where most of the mass in the Bullet Cluster is located [00:07:54]. The results showed that the majority of the mass is not in the middle where the gas is, but rather on either side [00:08:02]. This suggests that while the ordinary matter (gas) got stuck, the dark matter passed right through the collision without interacting, creating gravitational lensing where visible matter is sparse [00:08:07] [00:08:13].
Cosmic Background Radiation and Dark Matter
Additional strong evidence for dark matter comes from the cosmic microwave background (CMB), the oldest light in the universe [00:08:23] [00:08:37]. The CMB, which formed 380,000 years after the Big Bang, shows tiny temperature differences across the early universe [00:08:29] [00:08:41]. These temperature fluctuations can be analyzed by counting “blobs” of different sizes and plotting them as a graph with peaks [00:08:54]. The heights of these peaks are directly influenced by the amount of dark matter present [00:09:13]. To match the observed CMB measurements, the universe requires approximately five times as much dark matter as ordinary matter [00:09:27]. This estimation aligns consistently with the amounts of dark matter inferred from the motion of stars in galaxies and galaxies in clusters [00:09:34].
Nature of Dark Matter and Ongoing Research
The dark matter hypothesis offers a coherent theoretical framework that explains a wide array of different astrophysical observations [00:09:42]. It postulates the existence of a type of particle that interacts primarily through gravity [00:09:49]. While the exact nature of this particle remains unknown, scientists have proposed various candidates, such as Weakly Interacting Massive Particles (WIMPs) [00:09:57] [00:10:13]. WIMPs are expected to be roughly the mass of a proton but interact with ordinary matter extremely weakly [00:10:18].
Currently, numerous dark matter detection experiments are underway globally, attempting to directly detect these elusive particles [00:00:29] [00:10:03]. One notable experiment is DAMA/LIBRA, located under a mountain in the Italian Alps, which has been collecting data for about 20 years and shows a peculiar annual signal [00:00:41] [00:00:47]. This signal peaks in June and reaches a minimum in November, which some scientists suggest could be the first direct evidence of dark matter due to Earth’s varying speed through the galactic dark matter halo [00:00:55] [00:01:02] [00:02:01].
However, other similar experiments have not detected this signal, leading to uncertainty [00:03:19]. To resolve this, an almost identical experiment is being built in a gold mine near Melbourne, Australia, in the Southern Hemisphere [00:02:59]. Because the seasons are reversed there while the Earth’s motion through dark matter remains the same, a confirmation of the annual signal would provide strong evidence for dark matter’s existence [00:03:07].
The search for dark matter interaction with ordinary matter is paramount for understanding this mysterious component of our universe.