Professor Stephen Hawking investigates whether time travel is possible by looking at a range of possibilities.
Hawking considers the possibility of alien, intelligent life.
Scientists are still looking for the cause of the big bang and solve the elusive "theory of everything.".
Though dark matter makes up an estimated 99 percent of the universe, scientists still debate what it is.
Black holes are collapsed stars whose gravity devours matter.
Scientists investigate the relationship between matter and energy to uncover the essential building blocks of the universe.
The history of cosmology.
Changing views of the Earth's importance, from being considered the center of the universe 3,000 years ago, to a location in an undistinguished region of space.