Scientists have used Einstein's Theory of Relativity to calculate how time is measured in deep space flights. The conclusion is itself a paradox. For a deep space mission, the astronauts may age much more slowly traveling at the speed of light. For example, they may age 28 years on a light speed travel across the universe, but return to Earth 1,300 years later.
The fact that deep space travel can cause a return trip to take 1,300 years in "local" time explains why aliens have never returned to Earth.
Another factor in deep space travel is that human beings do not do well with forces over 1 g. We are basically water bags that can pop under pressure. Scientists think the best deep space travelers would be spiny, thin, small alien grays.
As private space firms have advertised in their speculative missions to colonize Mars, even solar system space travel is a one way street. If you plan to go out into deep space, the chance of your returning home is near zero.
Which brings us to a hypothesis in LOST.
Some believe the island may have been some lost alien space craft. If true, then Jacob, the island guardian would have been the alien pilot and his brother his co-pilot. If these space aliens came from a distant solar system with advanced technology, they could have crash landed on Earth without the means of repairing their ship.
The clues of being space aliens comes in the form of their physical presence. Jacob and especially MIB appear to be shape-shifting smoke monsters. They can change their physical structure, morph into human beings or monsters. This would make sense for deep space travelers to avoid the pitfalls of human bodies. If these alien travelers were not humanoids but energy beings, they could survive deep space flight.
A further clue on the island aliens is that Jacob's narrative that people who come to the island are from "shipwrecks." This mirrors his own situation. By bringing people to the island looking for a solution to his problem, Jacob cleverly puts the humans in the same position he is in: lost, looking for a way "home."
And the human beings brought to the island have been a clever bunch. Dharma and the military brought vast resources including nuclear technology and experimentation on electromagnetic properties including time travel. Perhaps these Earth technologies were being used to try to re-boot the space craft island so Jacob could indeed return "home" as MIB kept saying to the castaways. But MIB continues to get frustrated with the humans who want to take their advanced technology and use it to increase their own power on Earth. In order to control them, Jacob and MIB use human history of "gods" including borrowing ancient Egyptian rites, to control the humans on the island.
In essence, the series concludes not with the humans getting home, but the aliens finding a way to end their existence on a distant planet far away from their home. After centuries of failed attempts to get their ship technology working, Jacob and MIB apparently release themselves from their species protections (the immortality we saw during the show) in order to cease to exist. In that way, their alien technology does not corrupt the Earth society as they had found humans to be barbaric and crude in their emotional states.