Re'tu

The Re'tu are a fictional alien race from the sci-fi television show, Stargate SG-1.

The molecules of a Re'tu's body exist 180 degrees separated from our reality, so that they cannot be seen by humans and can only be interacted with physically to a limited degree (they can see humans just fine, however). They can however be exposed by radiation emitted by a TER device. When exposed, Re'tu appear as shadowy aliens with a humanoid torso, spider-like lower body, and insect-like head with two bug eyes on stalks. In many ways their appearance is similar to that of Species 8472 from Star Trek: Voyager.

Re'tu technology is fairly advanced, at the very least they are advanced enough to use energy weapons and be able to clone genetically engineered humans.

Because of their invisible nature, the Re'tu were deemed a threat by the Goa'uld, who attempted to exterminate them (Re'tu also emit a "vibe" that causes severe pain in a Goa'uld parasite, which allows Goa'uld and Jaffa to detect their presence). The aggression of the Goa'uld has split the Re'tu into two opposing factors: the majority Re'tu government who are non-hostile towards humans, and a large splinter group of Re'tu rebels who believe that the only way to fight the Goa'uld is to destroy all their potential hosts, including all human beings living in the galaxy. The rebels accomplish this with terrorist tactics aimed at killing as many humans as possible.

Re'tu fight with powerful arm-mounted energy cannons that burn large holes through flesh and metal. Because they are partially out of phase, Re'tu are highly resistant to physical weapons, which only slow them down. Re'tu are quite vulnerable to their own weapons as well as the energy beams fired by a TER device, and a single hit from either will cause them to explode messily.

The Re'tu only actually appear in one episode, but they are mentioned several times throughout the show. Whenever something bad happens in plain view yet no visible cause is apparent, the invisible Re'tu are usually amongst the prime suspects.