Science fiction is similar to fantasy, except stories in this genre use scientific understanding to explain the universe that it takes place in. It generally includes or is centered on the presumed effects or ramifications of computers or machines; travel through space, time or alternate universes; alien life-forms; genetic engineering; or other such things. The science or technology used may or may not be very thoroughly elaborated on; stories whose scientific elements are reasonably detailed, well-researched and considered to be relatively plausible given current knowledge and technology are often referred to as hard science fiction.