Wormholes are topological features representing shortcuts between two points in space-time. They can link two points separated by extremely vast distances, measured in light-years. Thus, a wormhole could be regarded as a tunnel, with the two entrances located at different points in space.
To simplify the notion, space can be visualized as a two-dimensional (2D) surface. In this case, a wormhole would appear as a portal through that surface, which led into a three dimensional tube (the inside surface of a cylinder), then re-emerged at another location on the 2D surface with a similar portal-like structure with respect to the entrance. An actual wormhole would be analogous to this, but with the spatial dimensions raised by one. For example, instead of circular outlines on a 2D plane, the entry and exit points could be visualized as spheres in 3D space, while the tunnel itself would be 4D.
Traversable wormholes were speculated to exist ever since the 20th century, and were confirmed to exist by the discovery of the Ring Network, which is comprised of more than a thousand stable wormholes.
The first wormhole encountered by humanity was the Sol Ring, part of an ancient, alien-made and still- functioning, network of wormholes leading to many star systems with habitable planets. The ring was created by the protomolecule, a pathogen created by an ancient, extinct civilization and launched in Phoebe towards prehistoric Earth to convert its biomass in order to build a gate and establish a wormhole leading to the slow zone. Instead, Phoebe was captured by Saturn's gravity and remained orbit around the gas giant for 1.5-2 billion years.
It wasn't until several eons later that the protomolecule would be discovered and subsequently released on Eros, where it consumed as much biomass as it could, almost killing Holden and Miller who were present there. With this amount of biomass, it was revealed that the protomolecule had the ability to create a hive mind of sorts, comprised of the consumed/absorbed residents of Eros Station and integrated with its computer systems.
Following the descent of Eros Station to the surface of Venus, the protomolecule began rapidly reshaping the planet. Observers throughout the Sol system observed these changes with great interest, but, despite the wealth of data, the objectives of the alien organism remained obscure.
The military science vessel Arboghast was tasked with observing Venus from orbit and relaying instrumental data to the UN. Eight months into her mission, the Arboghast received or emitted a brief radiation pulse of unknown nature and origin that interfered with all observation telescopes in the non-visible spectrum. Immediately following this, the entire vessel was disassembled into its structural components, down to the level of screws and bolts, methodically and with no discernible mechanism of action. The crew of 572, while not directly affected by the phenomenon, were killed by exposure to the Venusian atmosphere.
The protomolecule spent a little more than a year on Venus preparing itself, before it launched a massive complex of structures just outside the orbit of Uranus and built a thousand-kilometer-wide object known as the Ring. This is the final stage of the protomolecule life cycle: to convert itself to into a wormhole leading to the slow zone and make contact with the Ring Station.
Wormholes have been demonstrated to be feasible by the Ring Builders, who managed to construct an entire network connecting many systems. Although extremely stable, their physical capabilities can be overwhelmed if too much foreign matter attempts travel through them - generally referred to as "going dutchman." The overwhelmed wormhole would not collapse. Instead, it would strand all spaceships engaged in transit in its throat, thus in higher (4D) space, forever severing their link with known space. This astrophysical phenomenon was discovered by Naomi Nagata, who managed to weaponize this "weakness" and defeat the Free Navy. Even so, it is unknown exactly how very large quantities of foreign matter affect the wormhole and lead to it being overwhelmed.
Because of this danger, transit through the wormhole network is regulated by the Transport Union, as an attempt to avoid any gate from being overwhelmed.
Creation and stabilization
Although the exact process of creating a wormhole is almost wholly unknown, it is speculated to require the existence and use of unspecified exotic matter (matter with negative mass/energy) in order to hold it open and allow transit. A wormhole might also require some degree of quantum manipulation to ensure their survival over long periods of time.
The throat of the wormhole is the corresponding "tunnel" which links its two entrances through normal spacetime. The throat is theorized to be held open (and therefore traversable) by the existence and manipulation of exotic matter through unknown means. The length of the wormhole's throat is effectively zero, enabling the instantaneous transit of objects between the linked points.
When a wormhole's physical capabilities are overwhelmed, it is believed that the ships remain stranded in its throat.