Last week, Fringe finally gave us the conclusion to a weird ghost story that's been simmering since last season. But Fringe isn't the only science fiction universe to dabble in ghosts — there have been hauntings and apparitions in Star Trek, Doctor Who, Star Wars and countless other science fiction universes.

Just because there's a rational explanation doesn't make it less ghostly! Here's our list of spooky ghosts from science fiction.

Yui Ikari from Neon Genesis Evangelion
During a contact experiment with Evangelion Unit-01, Yui's body loses cohesion and reverts to LCL, the "primordial soup" from which all life on Earth is based. Her soul gets absorbed into the EVA's core. She doesn't quite go on living inside the EVA, it's more like she died and the EVA became her new body. And when Unit-01 appears to go berserk, it's actually Yui taking control.

Captain Kirk, Star Trek
In "The Tholian Web," Captain Kirk gets lost on an away mission — but crewmembers of the Enterprise keep seeing Kirk floating in mid air looking ghostly, including a great scene in Uhura's quarters, where she's wearing an insane nightie. Spock, being kind of a dick as usual, thinks that the crew is just seeing what they want to see. Until finally Kirk appears to the entire crew, on the Enterprise bridge, and Spock is forced to accept the evidence of his own senses — it's a transdimensional rift thing.

Optimus Prime/Optimal Optimus, Transformers: Beast Wars
This one gets a bit confusing. Beast Wars takes place millions of years in the past — after the Ark that brought the Autobots to Earth had landed, but before the Autobots inside had woken up. At the end of season two, the Maximals and Predicons find the Ark with all the Autobots, including Optimus Prime, in stasis. Megatron blasts Prime's body — and the resulting paradox causes a time storm to rage that would eventually erase the Maximals from existence (the Maximals being successors to the Autobots). Optimus Primal takes Prime's spark into himself to protect it, while Prime's body was repaired, and as a result Primal adopted his Optimal Optimus form, with added tank and jet parts. Primal replaced Prime's spark, but retained the new form. So basically, Prime died — and his soul lived on in Primal. It's not that complicated, really.

Zoe Graystone and Tamara Adama, Caprica
Zoe dies in a train explosion, but she lives on as a kind of ghost in the machine — both as a virtual entity, and later as a cyber-soul possessing a robot. She's also the first sort of proof of concept for the Soldiers of the One's idea of a virtual afterlife — except that lucky Zoe gets to have an afterlife and exist in this world. Also along for the ride: her fellow bomb victim, Tamara. (For more cyber afterlifes, click here.)

Ux-Mal criminals, Star Trek: The Next Generation
In the episode "Power Play" Troi, Data and O'Brien are taken over by entities that play themselves off as the spirits of a lost Federation ship. They actually turn out to be entities held prisoner in a penal colony on the same planet.

Pagh Wraiths, Star Trek: Deep Space Nine
What is it with Miles O'Brien and ghostly possession? He just has the worst luck. In the episode "The Assigment," Keiko O'Brien gets possessed by the spectral Pagh Wraiths, which want Miles to do make some wee adjustments to the station's systems for them — or else they'll use their control over Keiko's mind and body to cause some serious damage to her. Or possibly the kids. The Wraiths also possess people in a few other episodes.

Geordi LaForge and Ro Laren, Star Trek: The Next Generation
In another episode, "The Next Phase," the ship's engineer and the newest member of the bridge crew get apparently killed in an accident — but in fact, they're left out of phase with reality, leaving them invisible and intangible to the rest of the crew. (But they can still breathe oxygen and stuff. Best not to think about it too much.)

The Twins, The Matrix Reloaded
Remember these guys? The duo with white dreadlocks who could apparently noclip their way around the Matrix? They're another example of cyber ghosts. Maybe they could go on a double date with Zoe Graystone and Tamara Adama.

Obi-Wan Kenobi, Star Wars
Possibly the most famous ghost in science fiction. If you strike him down, he'll come back twice as strong, and three times as cryptic. Ghostly Obi-Wan is also partial to Yub-Nub parties. Put on some "Yub Nub" and he'll be there in a flash.

Space Ghost
We couldn't really leave out this interstellar superhero, who's basically like Green Lantern except with a ghost gimmick. As far as we can tell, Space Ghost is just an ordinary guy in a suit that gives him some powers that could be mistaken for ghostly activity. The Space Ghost FAQ is pretty fascinating reading, especially the part about how Space Ghost apparently doesn't have an origin. But he does have a great talk show.

Zoe, Morden and others, Babylon 5
In the fifth season episode "Day of the Dead," a ritual takes place during which several characters are visited by those who have recently died. A race called the Proceri rents out part of the station, because a passing comet makes the recently departed appear and visit their loved ones. Londo is visited by Adira, his old mistress and one true love who was poisoned by one of his rivals. Garibaldi is visited by an old flame named "Dodger" who I don't really remember. Captain Lochley gets visited by her friend Zoe, who has a message from Kosh for Sheridan. (In their youth Lochley and Zoe were involved with drugs or something, but Lochley managed to join Earth Force and turn her life around.) From an outside perspective the Proceri part of the station actually vanishes entirely from sensors. It's actually been transported someplace else entirely. Nobody's ever quite sure what's happened here.

The Gelth, Doctor Who
In "The Unquiet Dead," The Doctor and Rose encounter gaseous aliens who appear ghostly, and can take control of human corpses. They're actually a race who once had humanoid bodies but were turned into gas and displaced after the Time War. The Doctor gets the bright idea of letting them animate all the dead people in the Victorian era, but then realizes at the last minute why that might not fly.

Random Freedom Fighters, Doctor Who
"The Day of the Daleks" starts out being explicitly a bit of a ghost story, as the Doctor and Jo spend the night in a haunted house consuming mass quantities of wine and cheese — but the ghosts, as the Doctor points out, are actually from the future. They're guerillas who've come back in time to prevent the Daleks from conquering the planet, but their time machines are a bit wonky and they have a tendency to fade out.

Eugene Jones, Torchwood
In the episode "Random Shoes," Eugene Jones is killed — but his ghost still hangs around, stalking Gwen Cooper, because he swallowed an alien artifact right before he died. Eventually, Eugene is able to find peace and move on.

Psycho Mantis, Metal Gear Solid
Psycho Mantis, true to his name, is a powerful psychic — his boss fight in MGS1 is famous for tricking you into thinking your television is malfunctioning, plus the way Psycho Mantis "reads your mind" by looking at your memory card and seizes control of your controller, making it vibrate. After you kill Psycho Mantis in MGS1, his leftover psychic energy gets captured, and recreated using nanomachines (which are basically the answer to life, the universe, and everything in MGS.) This energy is used to control Screaming Mantis, who in turn keeps a psychic leash on the rest of the Beauty and the Beast Corps. After the boss fight with Screaming Mantis in MGS4, Mantis reappears in "spirit" form to say "hi."

The Sorrow, Metal Gear Solid
When The Sorrow was alive, he could use ESP to read the memories and assume the abilities of dead soldiers during a battle. He sacrificed his life to save his son, Adamska, in 1962. Throughout Metal Gear Solid 3, The Sorrow appears as a ghost in various scenes and Naked Snake eventually faces him in a sort of boss fight. During the fight with The Sorrow, Naked Snake must walk up a seemingly endless stream, where he encounters the ghosts of every single enemy that the player has killed up to that point in that playthrough. This can take a very long time, depending on how many people you've killed.

Almost everyone, Fringe
Who hasn't been a ghost on Fringe at this point? I guess Olivia and Walter haven't. Olivia's ex, John Scott, became sort of an apparition haunting her for a while. Walter's former partner in crime, William Bell, gets turned into a kind of ghost thanks to "Soul Magnets" and manages to possess Olivia. And then in the most recent episodes — spoiler alert — Peter Bishop is a ghost haunting both Walter and Olivia, until finally he turns into a blue blob of energy and manages to reincorporate himself in a lake. (Like a watery tart.)

Almost everyone, Lost
There's a bit of a J.J. Abrams theme going here — tons of characters got to be ghosts on Lost. Between Hugo Reyes' ability to see the dead, and the Man in Black's ability to impersonate some dead people, the cast of Lost was constantly running into apparitions. The young Man in Black also saw Claudia, who told him he was "special." Also, at one point it seems like anyone on the island can see ghosts — it just takes some people longer than others.

Zordon of Eltar, Power Rangers
A wise sage from the planet Eltar who fought against Rita Repulsa for two thousand years before being banished to a time warp, Zordon is still able to communicate with our dimension through a plasma tube. In the millennia that followed he befriended Alpha 4 and found Ninjor's first five Power Coins, which he bestowed upon the first generation of Power Rangers upon Rita's accidental reawakening. Aside from a flashback early in MMPR and the MMPR movie (which doesn't count) Zordon is always seen as a floating head in a tube.

Mallorie "Mal" Cobb, Inception
It sucks when your ex-wife keeps showing up and screwing up your game in the middle of a job — but it's way worse when she's dead, and it's her memory who's dogging you around. Mal keeps appearing as a sort of projection of Don Cobb's subconscious, creating a major logistical problem for Don and his team.

Anubis, Stargate SG-1
As a partially ascended being Anubis has some ghost-like qualities — and in one episode in particular, he actually possesses several people. In "Lockdown," it is revealed that after the destruction of his fleet over Earth, Anubis started moving as a black cloud between different human bodies, working his way to the SGC so that he can use the Stargate to escape back to his own territory. Throughout the episode Anubis jumps from person to person, wreaking havoc before finally leaving through the Stargate.