What Is Dark Matter? – With Guest Dr. Matthew Walker (Part 1)

February 27, 2013

John P. Millis, PhD and Jedidiah Becker for redOrbit.com – Your Universe Online

In the early 1930s, the Swiss astronomer Fritz Zwickey made a startling observation that would eventually become one of the greatest cosmological riddles of the twentieth century and beyond. While attempting to measure the mass of a large group of galaxies known as the Coma cluster, Zwickey found that the amount of mass calculated using gravitational theory was several hundred times greater than that predicted by measuring their brightness. Faced with this glaring discrepancy in data, Zwickey went on to theorize about the possibility of a type of unseen material in the universe which he dubbed “dunkle Materie” – or dark matter.

In the decades that followed, numerous experiments have reaffirmed the same fundamental paradox in galaxies all across the universe: According to current estimates, the amount of mass in the universe measured using gravitational theory appears to be about ten times greater than what we expect based on the luminosity, or brightness, of distant galaxies. Yet some 80 years after Zwickey’s bizarre discovery, astronomers are still divided as to the nature – and even the existence – of this dark matter.

Is it really possible that there are enormous amounts of material in the universe that somehow interact with the gravitational field but not with light? If so, what is this dark matter made of and where did it come from? Or was Einstein perhaps wrong, and our flawed view of gravity now has us chasing after phantom matter?

Which explanation is it? There is historical evidence to suggest that either is possible – and it turns out, this isn’t the first time that physicists have found themselves chasing after dark matter.

To further explore these questions Dr. Matthew Walker of the Harvard-Smithsonian Center for Astrophysics (CfA) spoke with redOrbit’s resident astrophysicist Dr. John Millis about the basic concept of dark matter and the two possible explanations for these observational oddities.

Be sure to listen to parts two and three of the interview, “The Search For Dark Matter” and “The Future Of Dark Matter Research.”


Dr. Matthew Walker is an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, and studies dark matter by measuring the motions of stars in galaxies. He has measured velocities for thousands of stars within the dwarf galaxies that orbit the Milky Way, and uses these data to infer the small-scale clustering properties of dark matter.

Dr. Walker earned a BS in physics and a BA in philosophy from Western Illinois University in 1999, and a PhD from the University of Michigan in 2007. He was a postdoctoral fellow at the Institute of Astronomy at the University of Cambridge, England, and is currently a Hubble postdoctoral fellow at Harvard College Observatory.