Alternity was an RPG system released just as the TSR/WotC switchover was taking place. I call it "The Sacrificial Lamb". For "various reasons" the system was dropped after just a few years. There were reports of "poor sales" and the like, but I think it was more about Wizards paring the TSR property down to one thing: Releasing a new D&D.
Alternity, as a system, wasn't bad. Alot of the underlying assumptions of the system were precursors to the changes from 2nd Ed to 3rd Ed. It had an "opposite" rolling system, however, where you sought to roll LOW, getting successes for rolling under your skill number. It was a "Leveled" skill-based system, which is to say, all the levels really did was give you a method for tracking how many skill points you should have. Everything was a skill, from handguns to "Resolve, Mental" which we would call a Will save in d20.
I have nothing against the system, and we actually play it about once a year or so. My brother-in-law and his friends really like it, so I tend to break it out when I go visit the in-laws in Cali.
Dark*Matter was one of the two full campaign settings released for Alternity. The other was Star*Drive, a "Space Opera" type setting.
Dark*Matter is a modern-day campaign setting similar in scope and theme to the X-Files. It was a little late on the draw for the full X-Files "explosion", but proved to at least garner a cult-like following (of which I'm a member). The PCs play agents of a non-governmental agency called The Hoffmann Institute, which investigates paranormal "stuff". UFO sightings, ghosts, alien abductions, psychic phenomena, etc. The PCs are supposed to investigate ... but standard issue equipment also includes a gun ... so yea. "The truth is out there ... and we shall shoot it."
In Dark*Matter the Greys are real and have been watching us for some time. In fact they have several city-ships, one on the dark side of the moon. Psychic abilities are real (you can even take a Wild Talent). Magic is on a comeback tour, and we're being invaded ... by about half-a-dozen different alien species who skulk about stirring up conspiracies to pave the way for bretheren (or maybe they just got stranded here and are trying to get home).
Agents are usually approached after something extraordinary happens to them ... thus rules for PCs who have wild talents, dabble in magic, or have been abducted. Of course your character may have had a run-in with government black-ops, the walking dead, or an angry demon.
I'VE always thought it was alot of fun ... but I'm biased.
--fje