Goldilocks
The universe as it is known is just a tweak or two away from never having existed. "Cosmologists in their lighter moments sometimes call this the Goldilocks effect." This metaphorical reference to the little girl who kept discovering a third option that was "just right" refers to the coincidental fact that the density of gravity is just perfect for creation of the universe. If the force of gravity were a little stronger, it would cause the universe to collapse and if it were a little weaker the universe would be infinitely larger but empty and inert. Luck or something like it has allowed gravity to attain the Goldilocks-like state of perfection between these two extremes.
Seeing a Supernova
Supernova is the term for a star which has exploded. The sight would be spectacular. "These would produce fabulous auroras, shimmering curtains of spooky light that would fill the whole sky." These metaphorical curtains of scary-cool sparkles would be quite a sight, true, but for a human to be close enough to enjoy this view would also necessitate instantaneous annihilation.
The Universe
"The universe, as Dennis Overbye has put it, is the ultimate sagging mattress." This metaphorical imagery refers to the fact that everything in the universe which has mass is analogous to a bowling ball placed on a bed. The weight of the ball is analogous to the mass of any object within the universe and that mass has the ability to leave an impression on the fabric of the universe.
Atomic Resurrection
Atoms that make up every single individual have been around for billions of years. "So, we are all reincarnations - though short-lived ones." This metaphorical concept of reincarnation is based on the likelihood that so many of the atoms which make up each person have previously made up other people in the past. One estimate posits that every human alive today shares a billion atoms that once constructed Shakespeare and Genghis Khan, among others.
Characterization
A brilliant scientist named Henry Cavendish is summed up using a metaphor. "Cavendish is a book in himself." This characterization works on two level. The first refers to the fact that Cavendish led such a full and varied life. The secondary utilization here is that this full life of Cavendish could literally be worth an entire book written by the author of this book.
