My delay is in no way a reflection of my opinion of Dark Cosmos, as I'm quite favorably disposed toward it. This past Thursday, I attended a physics colloquium at Texas Tech pertaining to dark energy (given by Gary Hill of the University of Texas, Austin) and that's motivated me to write again about the book. So, here's my review.
Dark Cosmos introduces dark matter and dark energy for a general, well-educated audience, with both clarity and relative brevity (a little over 200 pages). Even though I'd read about the book's two key concepts previously, I was still a little bit in the "dark" about them, so reading the book was a welcome learning experience. For those who might want to consider reading the book, an excerpt of it from Smithsonian magazine is available here.
Hooper writes considerably more about dark matter than about dark energy, given that the former has been studied for longer and more is known about it, than the latter. As a way to compile the main ideas from the book in an easily digestible format, I made the following chart (which you can click on to enlarge).
The initial chapters of the book, which are on dark matter, are written almost in detective-novel style. Possible candidates for what might comprise dark matter are discussed -- and cast in doubt -- sequentially. One class of objects that had been considered candidates are:
...massive compact halo objects, or MACHOs. The group includes dead stars like white dwarfs, stars that never burned like brown dwarfs, strange exotic stellar entities like neutron stars and black holes, and large Jupiter-like planets... (p. 36).
On p. 41, however, Hooper reviews research suggesting that MACHOs are not plentiful enough to constitute the dark matter, rendering them "rather poor candidates..." As an aside, I found Hooper's tutorial on the life cycle of stars and related entities to be very helpful.
Other dark-matter candidates discuss include neutrinos and supersymmetric superpartners.
The book then shifts to dark energy, which in turn sparks a discussion of the Big Bang and inflationary cosmology. The latter concepts are fascinating in their own right, so I think I'll write a separate entry about them in the future.
As Hill alluded to in his Texas Tech talk, Science magazine, upon its 125th anniversary, identified 125 outstanding scientific questions. Heading the list is the question of "What Is the Universe Made Of?," which implicates dark matter and dark energy. For an excellent introduction to these concepts, I highly recommend Hooper's Dark Cosmos.