In 1933, Fritz Zwicky (California Institute of Technology) made a crucial observation. He discovered the orbital velocities of galaxies were not following Newton’s law of gravitation (every mass in the universe attracts every other mass with a force inversely proportional to the square of the difference between them). They were orbiting too fast for the visible mass to be held together by gravity. If the galaxies followed Newton’s law of gravity, the outermost stars would be thrown into space. He reasoned there had to be more mass than the eye could see, essentially an unknown and invisible form of mass that was allowing gravity to hold the galaxies together. Zwicky’s calculations revealed that there had to be 400 times more mass in the galaxy clusters than what was visible. This is the mysterious “missing-mass problem.” It is normal to think that this discovery would turn the scientific world on its ear. However, as profound as the discovery turned out to be, progress in understanding the missing mass lags until the 1970s.

In 1975, Vera Rubin and fellow staff member Kent Ford, astronomers at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington, presented findings that reenergized Zwicky’s earlier claim of missing matter. At a meeting of the American Astronomical Society, they announced the finding that most stars in spiral galaxies orbit at roughly the same speed. They made this discovery using a new, sensitive spectrograph (a device that separates an incoming wave into a frequency spectrum). The new spectrograph accurately measured the velocity curve of spiral galaxies. Like Zwicky, they found the spiral velocity of the galaxies was too fast to hold all the stars in place. Using Newton’s law of gravity, the galaxies should be flying apart, but they were not. Presented with this new evidence, the scientific community finally took notice. Their first reaction was to call into question the findings, essentially casting doubt on what Rubin and Ford reported. This is a common and appropriate reaction, until the amount of evidence (typically independent verification) becomes convincing.

In 1980, Rubin and her colleagues published their findings (V. Rubin, N. Thonnard, W. K. Ford, Jr, (1980). “Rotational Properties of 21 Sc Galaxies with a Large Range of Luminosities and Radii from NGC 4605 (R=4kpc) to UGC 2885 (R=122kpc).” Astrophysical Journal 238: 471.). It implied that either Newton’s laws do not apply, or that more than 50% of the mass of galaxies is invisible. Although skepticism abounded, eventually other astronomers confirmed their findings. The experimental evidence had become convincing. “Dark matter,” the invisible mass, dominates most galaxies. Even in the face of conflicting theories that attempt to explain the phenomena observed by Zwicky and Rubin, most scientists believe dark matter is real. None of the conflicting theories (which typically attempted to modify how gravity behaved on the cosmic scale) was able to explain all the observed evidence, especially gravitational lensing (the way gravity bends light).

Currently, the scientific community believes that dark matter is real and abundant, making up as much as 90% of the mass of the universe. However, dark matter is still a mystery. The most popular theory of dark matter is that it is a slow-moving particle. It travels up to a tenth of the speed of light. It neither emits nor scatters light. In other words, it is invisible. Scientists call the mass associated with dark matter a “WIMP” (Weakly Interacting Massive Particle). However, the WIMP particle is speculative and to date has not been proven to exist. In addition, it is not predicted by the standard model of particle physics. (Some physicists have performed reformulations of the standard model to have it predict the WIMP and other particles. However, none of the particles predicted by the reformulated standard model have ever been verified.)

There is little doubt, though, that dark matter is real. There experimental evidence is solid. The rotation of stars, planets, and other celestial masses orbit galaxies, like ours, too rapidly relative to their mass and the gravitational pull exerted on them in the galaxy. For example, an outermost star should be orbiting slower than a similar-size star closer to the center of the galaxy, but we observe they are orbiting at the same rate. This means they are not obeying Newton’s laws of motion or Einstein’s general theory of relativity. This faster orbit of the outermost stars suggests more mass is associated with the stars than we are able to see. If not, the stars would fly free of their orbits, into outer space.
We can see the effect dark matter has on light. It will bend light the same way ordinary matter bends light. This effect is gravitational lensing. The visible mass is insufficient to account for the gravitational lensing effects we observe. Once again, this suggests more mass than what we can see.

We are able to use the phenomena of gravitational lensing to determine where the missing mass (dark matter) is, and we find it is throughout galaxies. It is as though each galaxy in our universe has an aura of dark matter associated with it. We do not find any dark matter between galaxies.

While there is no doubt that dark matter is real, its nature remains a mystery. Is it a particle? Is it a new form of energy? All effort to detect the WIMP particle over the last decade or so have been unsuccessful, including considerable effort by Stanford University, University of Minnesota, and Fermilab. Where does this leave us? The evidence is telling us the WIMP particle might not exist. We have spent about ten years, and unknown millions of dollars, which so far leads to a dead end. This appears to beg a new approach.

To kick off the new approach, consider the hypothesis that dark matter is a new form of energy. We know from Einstein’s mass-energy equivalence equation (E = mc2), that mass always implies energy, and energy always implies mass. For example, photons are massless energy particles. Yet, gravitational fields influence them, even though they have no mass. That is because they have energy, and energy, in effect, acts as a virtual mass.

In my book, Unraveling the Universe’s Mysteries, I suggested an approach to test the hypothesis that dark matter may be a new form of energy. Because of the length of discussion necessary to describe my suggested approach, I will not go into it in this post. My main point in this post is to suggest we widen our investigation into the nature of dark matter to include the hypothesis that it may be a new form of energy. As a scientist, I think we have to broaden our search. I acknowledge it is possible that dark matter may be a WIMP particle, but we have no conclusive evidence after over ten years of research. Therefore, we should widen our search to include the hypothesis that it is a new form of energy.