While Randall's main focus is on theories involving extra-dimensions, she explains a lot of related topics along the way, and for me, one of the most striking details she reveals has to do with the varieties and relative proportions of matter and energy in the Universe. Randall explains...
"Dark Matter is the nonluminous matter that pervades the universe and has been discovered through its gravitational influence. Even though about one-quarter (25%) of the energy in the universe* is stored in dark matter, we still don't know what it is...By contrast, ordinary matter from which we are made of makes up the remaining five percent (5%).
...The universe contains dark energy** (energy not carried by any matter), that constitutes seventy percent (70%) of the total energy of the universe...No theory [is able to] explain dark energy."
Update April-20-2008: Via 3Quarksdaily, an experiment that claims to detect Dark Matter in the form of Weakly Interacting Massive Particles (WIMPs).
Also, here's the web page for NASA's Wilkinson Microwave Anisotropy Probe (WMAP) that i mentioned in the comments section. It has further interesting information including this Timeline of the Universe.
In the above diagram, the acceleration of the Universe's expansion due to Dark Energy is noticeable.
Update April-25-2008: Over at Cosmic Variance, blogger-physicist Sean Carroll explains that the famous equation e=mc2 does not apply in the case of Dark Energy.
"Interestingly, the dark energy that makes up 70% of the energy of the universe doesn’t really have “mass” at all, since it’s not made up of objects (such as particles) that can have a rest frame — it’s a smooth field filling space."*Matter and energy is intergchangeable via e=mc2.
**This mysterious Dark Energy is also known as Vacuum Energy and was first proposed by Einstein as the Cosmological Constant and more recently theorized as a new substance called Quintessence. Its presence is inferred by the observation that the expansion of the universe seems to be accelerating.