I just finished reading the book Einstein's Telescope: The Hunt for Dark Matter and Dark Energy in the Universe, by Evalyn Gates (website for book). Her objectives are clear: to understand exactly what dark matter and dark energy are, using the tool of gravitational lensing. A bit of background information may be in order for some readers. To brush up on dark matter and dark energy, there's my review of Dan Hooper's book Dark Cosmos and a previous write-up of mine regarding a talk I attended by John Moffat.
Some background on Einstein's theory of general relativity, which gives rise to gravitational lensing (hence the name of Gates's book), might also be helpful for some. According to general relativity, gravity is the warping (indenting) of spacetime's fabric by heavy objects so that other objects fall toward them. Gravitational lensing refers to how indented areas of spacetime can bend light that is traveling by, thus creating optical effects. A visual illustration and brief explanation of gravitational lensing are available on this page from Lawrence Berkeley Laboratory.
Dark matter -- by definition -- cannot be seen directly. However, to the extent lensing effects involving other light sources (e.g., stars or quasars) can be observed where there is no visible matter present to exert a lensing effect, that effect is inferred to be due to dark matter. Two possible classes of candidates for dark matter are known as MACHOs (Massive [Astrophysical] Compact Halo Objects) and WIMPs (Weakly Interacting Massive Particles). Gates notes that MACHOs are "things that we already know exist, such as planets, faint stars, and black holes," whereas WIMPs are postulated to be "entirely new kinds of particles" (p. 97). I don't recall seeing MACHOs and WIMPs distinguished in this way, so I appreciate the clarification.
Gates describes one instance of using gravitational lensing to try to infer the presence of dark matter, as follows:
A typical MACHO mass... is much smaller [than an entire galaxy] -- about half the mass of the Sun. Because a less massive object will cause a shallower warp in the surrounding spacetime and a correspondingly smaller deflection of light, gravitational lensing by a MACHO results in a much more subtle shift in the path of the light from a distant star. The star must be almost exactly behind the MACHO in order for the star's light to be noticeably affected by the small dimple in spacetime caused by the MACHO (pp. 101-102).
Regarding dark energy, which is understood even less well than is dark matter, Gates notes that gravitational lensing cannot be used to make the same kind of fairly straightforward inferences about dark energy the same way it can about dark matter. "And we can't even try to find a lump of [dark energy] using gravitational lensing -- dark energy doesn't clump like mass, but appears to more or less evenly permeate the cosmos" (p. 225).
Instead, as she describes, "...two cosmological probes of dark energy -- changes in the expansion history of the Universe, and changes in when and how quickly structures were formed -- are related to one another in a Universe filled with dark energy" (p. 226). Further, "[t]he main ingredient in the recipe for making such a virtual model of the Universe is dark matter" (p. 228) and "[t]he ultimate lens of the Universe is the dark matter web" (p. 234). Finally, "[t]he goal of dark energy lensing experiments is to reconstruct this web so that we can study it carefully and extract the imprint of dark energy" (p. 234).
In sum, therefore, gravitational lensing ("Einstein's Telescope") may serve astronomers and physicists well as a tool for understanding both dark matter and dark energy. Gates lists several proposed telescopes and experiments (some with scheduled launch dates) that may be able to make and report new discoveries regarding dark energy over the next two decades. Lensing approaches are part of some of these proposals.
I found the first few chapters of Einstein's Telescope pretty accessible. As the book went on, however, I began to feel it was reaching a technical level above my working knowledge and providing extensive detail that was hard for me to follow. On the positive side, the book contains a number of helpful diagrams and engaging color photographs of space. For a reader such as myself -- well-educated, but without any advanced training in physics, other natural sciences, or mathematics -- I would recommend Einstein's Telescope only for individuals who have already read previous physics books intended for well-educated general audiences, such as Brian Greene's The Fabric of the Cosmos or the aforementioned Dark Cosmos.
[Lightly edited for clarity and typos on September 25.]