As with reflex technology and anything found in the facilities on Sèlene, we have no idea where it came from, who built it, what interest they might have had in the Earth, or where they went. Our best scientists estimate that The Ancients abandoned their facilities on the Sèlene millions of years ago and we have found nothing bearing their hallmarks on Gaia.
They have, without a doubt, opened the stars to the human race. Sentinels are vast, monolithic structures made of a material unknown to science thought it appears to be some kind of living metal that can repair itself from comet and asteroid strikes. We also know that Sentinels can tear open the fabric of space-time.
In its rest state, a Sentinel is one; a seamless piece of whatever it is, floating in space like, well, a sentinel on guard. Only when it receives the correct code, randomly generated using a complex matematikal sequence that the dwarves and Albrecht Aìnstân managed to crack centuries ago, does the Sentinel begin its stately separation into four equal pieces and, as it does so, space is ripped open between these pieces, creating a wormhole. After that, one has simply to convert ones destination coordinates into the same complex code and, at that destination, another Sentinel will separate. Transit between the two Sentinels down a stable and shielded wormhole rarely takes longer than a few hours through whatever medium it is that the ship travels. The longest test flight took an unpiloted Harpy from Earth to the Eastern Galactic Rim and back in six months of local time yet the Harpy's chronometers had logged only two weeks.