Boy of Age 16 Asks Me about Relativity, etc.6. What Is Dark Matter?

Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689 indicates the presence of dark matter (Ref. 1). Image by NASA, N. Benitez (JHU), T. Broadhurst (Racah Institute of Physics/The Hebrew University), H. Ford (JHU), M. Clampin (STScI), G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the ACS Science Team and ESA [Public domain], via Wikimedia Commons

A friend of mine on Twitter, Aaron (a pseudonym), is an overseas, 16-year old boy, who seriously admires Albert Einstein and wants to become a physicist. He continually writes me (Ted, also a pseudonym) questions about the theory of relativity and related topics, and I am sending answers. In this series of blog posts, those questions and answers are reproduced with modifications. I am not an expert in the fields of physics related to relativity. So, my answers might contain errors. If you find any error, please do not hesitate to write a comment for the benefit, not only of the boy and me, but also of other readers.

Aaron: What is dark matter? How did they know about it?

Ted: It is difficult for me to give my own, accurate description of dark matter. So, here I write a summary of the excellent Web page written by Professor Martin White at Lawrence Berkeley National Laboratory (Ref. 2).

Dark matter is "stuff" which cannot be seen directly. Its existence is inferred indirectly from observations of motions of astronomical objects, specifically, stellar, galactic, and galaxy cluster/supercluster observations. The basic principle of these observations is that if we measure velocities in some region, then there has to be enough mass there for gravity to stop all the objects flying apart. Such measurements done on large scales indicate that the amount of inferred mass is much more than can be explained by the luminous stuff. Hence, we infer that there is dark matter in the Universe.

Dark matter candidates are usually divided into two broad categories, with the second group being sub-divided:

• Baryonic*
• Non-Baryonic
• Hot dark matter and
• Cold dark matter.

For more details, see Ref. 2 and links there.

* Baryonic matter is matter composed mostly (with regard to mass) of baryons, which in turn are composite particles made up of three quarks, and includes atoms of any sort.

References

1. "Dark Matter," Wikipedia, The Free Encyclopedia (2 January 2012 at 17:44).
2. Martin White, Dark Matter.

(Originally written on March 2, 2011)