Peter Fisher

What we know about dark matter and what we have yet to discover Astronomical observations have confirmed dark matter’s existence, but what exactly is dark matter? In What Is Dark Matter?, particle physicist Peter Fisher introduces readers to one of the most intriguing frontiers of physics. We cannot actually see dark matter, a mysterious, nonluminous form of matter that is believed to account for about 27 percent of the mass-energy balance in the universe. But we know dark matter is present by observing its ghostly gravitational effects on the behavior and evolution of galaxies. Fisher brings readers quickly up to speed regarding the current state of the dark matter problem, offering relevant historical context as well as a close look at the cutting-edge research focused on revealing dark matter’s true nature. Could dark matter be a new type of particle—an axion or a Weakly Interacting Massive Particle (WIMP)—or something else? What have physicists ruled out so far—and why? What experimental searches are now underway and planned for the near future, in hopes of detecting dark matter on Earth or in space? Fisher explores these questions and more, illuminating what is known and unknown, and what a triumph it will be when scientists discover dark matter’s identity at last.

What Is Dark Matter?

Amira BEN

Dark matter and dark energy may be in some way related, but no one yet knows how, and the effects of the two forms of matter-energy are very different. 

Dark matter shapes galaxies and clusters of galaxies, and it helps dictate how the universe expands. 

Dark energy affects the expansion rate of the universe; and the current data imply that dark energy's effects are the same everywhere in the universe and that dark energy acts as a cosmological constant, meaning dark energy's pressure is the opposite of its density.

In Einstein's theory, the observed galaxy not moving 

 space; instead, space expanding, carrying the galaxy with it.

The cluster as a whole follows the expansion driven by dark energy, called the Hubble flow. The galaxy cluster does not change in size and moves along with the Hubble flow as viewed by an observer on Earth.

, relates the average kinetic energy of the cluster with the cluster's potential energy. 

Having measured the average and spread in velocities, the virial theorem then gives the mass of the cluster, so Zwicky could form a ratio of mass-to - light output for the cluster. The ratio came to about 400-500 solar masses per solar luminosity. 

Observation of Kaptyn's star gave a mass-to-light ratio of about 3 solar masses per solar luminosity, leading Zwicky to conclude fraction of the matter in galaxies is not light-emitting stars in the galaxies.

 The bottom line: Not enough baryonic matter came out of the Big Bang to account for all the visible and dark matter observed in galaxies and clusters of galaxies, so what we call dark matter cannot be failed stars, planets, anything else made of baryons.

At the same time, modifications to Newton's second law proved untenable. Where does that leave us? The conclusion is that our understanding of particle physics is incomplete, and we will need to add something to the Standard Model to explain dark matter.

 The annihilations also should produce energetic photons or gamma rays. Extra positrons, anti-protons, or gamma rays in cosmic rays could be an indication of dark matter annihilation taking place in the center of the Milky Way.

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