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 continue to explore the big-bang theory.
Galaxies and stars account for one percent of all matter.
Black holes are collapsed stars whose gravity devours matter.
The relationship between matter and energy.
The big-bang theory suggests the universe began with an explosion.
Scientists attempt to understand the obscurity of the universe.