For most of history, the existence of alternate, or 'parallel,' universes has been nothing more than a scientific theory and vague possibility. In 2007, however, alternate universes became a fact, although most of the world still lacks hard proof of their existence.
Detailed knowledge of Alternate Universes is largely limited to individuals with high security clearance or a serious case of wrong place wrong time. In particular, X-Force dealt with a number of AUs over the 31 years of its existence. Some members of the US military and a small group of scientists also have first-hand knowledge, as do a handful of civilians.
Several AU rifts have opened in other countries. It's not clear what their governments do or do not know about AUs; they have by and large been kept quiet, however, and no government or organization has publicly claimed the existence of AUs. It seems likely that other countries, like the United States, have limited the knowledge of such rifts to personnel bound by NDAs.
Despite this, there is a rabid following of underground rumors of the existence of Alternate Universes. There are popular videos which claim to show such universes, but they are considered by most to be hoaxes created with a mix of practical and advanced CGI effects. Claiming to believe in AUs is a quick way to be labeled a conspiracy nut.
The underlying causes of the rifts and the function of alternate universes remains unknown. Although there have long been a number of scientific theories, including the Multiverse and the many-worlds interpretation of quantum physics, none have yet been concretely proven. For a time, a heavily-funded center for research on rifts, alternate universes, and the quantum physics which may cause them existed at Tompkins Square Park, on the site of the first rift. It was dismantled in 2027, and the park was re-opened to the public.
A top-secret, state of the art government research facility on Alternate Universes was established in Glen Haven, Colorado in 2025. A lesser research location has been present on Staten Island since 2040