Pocket Universes are often believed to be much smaller than a conventional or mature universe. While very little hard data or reproducible data is available, theorists have widely asserted that this type of cosmology exists.
An Alternate Dimension is conjectured to be a territory of sequestered spacetime located outside of the local universe. Alternate Dimensions cannot be reached using conventional technologies. And the very phenomenon is not yet verifiable or well understood. Some believe an alternate dimension to be widely equivalent to a pocket universe while others feel that if the astrophysical constraints differ then an alternate dimension cannot be a pocket pinched off of the baseline universe.
Whatever one may or may not be, the phenomenon mostly exists as theoretical science.
Pocket Universe Origins
From whence do pocket universes come? No known scientist within Charted Space seems to definitively know.
Some theorize that a pocket universe may be part of a natural phenomenon, perhaps an infant or baby universe... Perhaps universes follow a pattern of development comparable to sophont-level lifeforms?
Whatever the truth may be, pocket universes are a hot topic of study within Charted Space. Where even the slightest evidence of a pocket universe appears, scientists, adventurers, and travellers will flock.