A subspace rift
A subspace rift is a rupture of a region of subspace instability where subspace extrudes into normal space due to excessive exposure to warp field energy. Such a phenomenon is characterized by tetryon radiation and high-energy distortion waves, and is extremely sensitive to the exposure of further warp field energy.
In 2370, Hekaran scientists Serova and Rabal began mining the Hekaras Corridor with verteron probes to disable warp-powered vessels, in an attempt to gain the attention of the Federation and press their case that cumulative damage being done in that area of space was causing severe ecological damage to their homeworld. When the USS Enterprise-D arrived to investigate, Serova and Rabal presented their case, but were rebuffed when it appeared that they did not have enough hard evidence. In response, Serova initiated a warp core breach on her vessel, exposing a region of subspace instability to a massive amount of warp field energy in order to create a rift, sacrificing her life to prove her theory. Unfortunately, the rift enveloped the medical transport vessel Fleming, stranding them inside. The crew of the Enterprise was forced to develop a plan whereby they could rescue the Fleming without active use of their warp engines, as exposure to further warp field energy would cause the rift to expand even farther. Lieutenant Commander Data came up with the idea of initiating a high-intensity warp pulse to "coast" into the rift. While the Enterprise was inside the rift, the Fleming attempted to initiate their warp drive, an action which caused the rift to expand. After beaming the crew of the Fleming aboard the Enterprise, they were able to escape the rift by phase-matching their deflector shields to the EM variance of the distortion waves, riding the wave out of the rift.
Subsequently, Rabal and Geordi La Forge's scans showed a lot of subspace instabilities throughout the sector that could eventually create other rifts if they were exposed to enough warp energies. Projections of the probable locations where subspace rifts would form over the next forty years based on current warp drive patterns had them occupying nearly all of the sector. In order to slow the damage, the Federation Science Council ordered that areas susceptible to subspace rifts would be restricted to essential travel only with a speed limited to Warp 5 except in cases of extreme emergency. The Federation would share their data with all warp-capable species in the hopes that they would realize that it was in their own interest to take similar action. However, there was nothing that they could do about the rift that had already formed and the gravitational shifts from it began to affect Hekaras II's orbit and cause climate change. The Federation set up a weather control matrix on the planet as a temporary solution in the hopes of eventually finding a more permanent solution for the rift and its effects. (TNG: "Force of Nature")
In 3190, Commander Jett Reno realized that the destruction of Species 10-C's hyperfield would kill everything within it and leave a subspace rift near Earth that would kill everyone there nearly as fast as the Dark Matter Anomaly. When confronted, Ruon Tarka claimed that Earth would have a month before the rift impacted, giving their scientists plenty of time to find a solution. (DIS: "Species Ten-C")
After the DMA was dismantled, the USS Discovery kept track of the subspace rifts left behind by it and determined that the 10-C were keeping their promise to clean them up. Ensign Adira Tal, while monitoring the work with Linus, reported that it was going faster than expected. (DIS: "Coming Home")