The so-called Microverse is an extra-dimensional realm only marginally connected to Earth. The primary means of accessing the Microverse appears to be through the shunting of mass extradimensionally via Pym Particles, although more traditional methods (such as vibrational attunement) might still be used to transition between the realms.
The Microverse as a whole has not been mapped, but it appears to have laws of physics similar to our own. Stars and planetary systems exist in the Microverse, though it is not known whether the stars coalesce into galaxies or are spread fairly evenly throughout the universe. One star system in the Microverse appears to possess roughly the same "location" in that universe that Sol does in ours; it is this system that has had the most contact with Earth.
This star system has four habitable planets. One planet, K'ai, is home to a race of green-skinned humanoids roughly fitting the human physical norm, and has a level of technology roughly equivalent to Europe's High Medieval Age. The other three planets – Zebulon, Cadmus, and Azara – have technology slightly ahead of Earth enabling regular travel between them, and have formed the Alliance of United Worlds (simply called "the Alliance" in everyday speech). The people of Zebulon and Cadmus appear to be human, while the people of Azara are humanoids with skin tones that vary between scarlet and lavender. The people of Zebulon claim to be descended from a "lost tribe" of Earth humans from some time in the past; the people of Cadmus are descended from those of Zebulon who rejected the claim of descent from another realm and were exiled to their sister planet. Zebulon and Cadmus are roughly the same size and orbit around a point roughly halfway between the two. Azara and K'ai are "co-orbital" in that their orbits are close enough to each other that the planets occasionally swap orbits when they get close to each other, roughly once every year and a half.
A nearby system contains the planets Terragonia, home of an green-skinned insectoid people ruled by a Queen, and Bast, home of a black-furred feline humanoid people. Terragonians are often found among the people of the Alliance; most likely, the systems are part of a binary system, permitting travel between them.
The first people from Earth to visit the Microverse were Hank Pym and Janet van Dyne, the adventurers known as Ant-Man and Wasp, who visited Azara following a mishap involving the size-change inducing Pym particles. Not long after, the planet Zebulon was visited by the Fantastic Four, who were investigating whether the Dr. Pym's accident with Pym Particles could be duplicated.
Contact between Earth and the Microverse was non-existent for months, until Mister Fantastic, leader of the Fantastic Four, mistakenly transported the Molecule Man to the Microverse instead of another universe he was aiming for. The Molecule Man formed a partnership with a renegade technologist who had taken to calling himself the Psyho-Man, after developing psychotronic technology which would create and amplify various emotional states. The two lured the Fantastic Four back to the Microverse and attempted to turn them against each other. The two were defeated and apparently dissolved their partnership. The Psycho-Man has since been spotted on Earth; due to various factors, the Psycho-Man appears as only one foot tall on Earth. It is not known whether the one-foot height is the maximum height all Microversians can attain on Earth or if he deliberately chose to be five and a half times smaller than normal.
The Alliance has since come under attack from forces under the command of the renegade Azaran calling himself General Kartha, who intends to conquer all four worlds. To combat Kartha's forces, the Alliance has formed an elite strike team, which has not yet been named but consists of the best warriors in the Alliance, led by the Alliance Colonel Arcturus Rann of Zebulon and Princess Mari of Cadmus.
First Post-Reboot Appearance: ASTONIGHING ANT-MAN #.
K'ai is a swords-and-sorcery world with a native TL3, and in a campaign set in the modern day, visitors from K'ai to Earth would have Low TL -5 [-25]. The other planets have a native TL 10, with limited superscience (mostly in artificial gravity), and would have High TL 2 [10] when visiting Earth.
1. I really didn't want to go with names such as "Sub-Atomica" or "Micronauts", as the Microverse is not actually a "sub-atomic" realm, nor would its people believe themselves to be part of such. It's a case of "you don't have to get your science 100% correct, just make sure it's not blatantly wrong!" To call it a "sub-atomic" realm is clearly "blatantly wrong" in my opinion.
2. Not sure yet what the best name for the reworked Micronauts is, or how to present them. Marvel cannot legally use the name "Micronauts" any more, nor use half the members of the Micronauts team used in the title, due to the Micronauts originally being a licensed title (much like other licenses they've had, such as Star Wars, Transformers, ROM, and G.I.Joe) of a toy line. Bug, Marionette, and Arcturus Rann were Marvel creations and hence owned by Marvel, so they're usable; the rest of the team and Baron Karza, their nemesis, are not. The current official name (as of the initial writing of this entry) of the team is "Microns", but that conflicts with my previous note. (General Kartha is, of course, an expy of Baron Karza with a slight name change for copyright and trademark issues.)
2a. Addendum: Recently Marvel seems to be using the name "Omega Force" for a bunch of Micronauts-related stuff. Seems as good a name as any.