Possibly the single greatest discovery in the last three hundred years was made by Dr. Robert Lilis in his experimentation with gravity, learning to simulate it. In the process, he discovered a reproducible method for creating traversable wormholes, generally called Lilis Wormholes.
It was known for some time that, by funneling incredible amount of radiation to a single point, it is possible to warp space-time enough to create a singularity, a point where spacetime tears. Once such a the wormhole is created, it collapses almost immediately… but the tear in spacetime remains, waiting for sufficient energy to be supplied to reopen it again. This is usually provided through kinetic energy – a ship passing through a wormhole would require a enormous amount of speed and mass that the wormhole would absorb to become traversable. This process would rob the ship of all its velocity, leaving it at a standstill on the other side of the wormhole as it collapses again. For this reason, only ships with exceptionally powerful engines or incredible mass, enough to generate enough energy to approach the speed of light, would be able to use one.
Dr Lilis’ discovery was a way to circumvent this. Through creating another of these wormholes and direct gamma radiation at it from the main engines, it allow a process called superradient scattering to dramatically increase the scope scope of that energy… and energy generate, even once bled of, has mass. It allows the ship to “cheat” the amount of mass it has for the purpose of delivering kinetic energy and make it through one of the wormholes before the bridge collapses.
Now, simply being able to pass through one isn’t, in and of itself, especially useful as the Lilis wormholes are created in the same space as one another, the two singularities emerging at the same point. However, they are essentially tiny but enormously heavy objects… they can be picked up and moved. Without them being active, they don’t generate much gravitational pull or have an event horizon, but the amount of energy that went into the rip in spacetime still has a mass of its own, an Einstein predicted in the earth 20th century.
Earth has taken to crafting these Lilis wormholes and then sending out exploration ships towing their twin along. The end affect of this is that while it might take years or decades to reach a system for the first time and deliver the Lilis Wormhole into orbit around the Sun, the second time travel is nearly instantaneous. This has lead to a slow/fast expansion of the Terran Federation, where the sphere of human control expands only slowly but the rate of growth and colonization within it has begun to accelerate greatly.
As a side benefit, this extra mass also generates gravity. By locating the drive on bottom deck of a starship, it ensures that no matter how it faces, towards the Lilis drive always feels like “down.”
The process by which radiation can be funneled to make this kind of zero-matter singularity was discovered by freak accident while experimenting on something else, and none of the prerequisite technologies or theories are essential for interstellar travel. Human scientists expect that if there are other space-faring aliens out in the galaxy, it is by no means assured or even likely that they have discovered the same process yet. In fact, by the fact that no species has contacts mankind yet and a form of faster-than-light travel exists, it seems very likely no one else has discovered this and humanity is the first.