These are naturally occurring Gravitational Anomalies. They attract Entities and break blocks. Each anomaly has it’s own mass and can feed on entities near its event horizon. Matter from each entity is added to the anomaly’s mass. Once the anomaly becomes massive enough it begins destroying the terrain around it. The radius of the anomaly can be seen on the Gravitational Stabilizer used to stabilize it, or can be calculated using Newton’s inverse-square law. The further the block is, the less likely it is to be broken and consumed.
The hardness of the block determines if the block will be broken by the anomaly. Dirt or Redstone will be broken almost immediately but Obsidian will be broken by only the largest of anomalies.

Gravitational Anomalies are also the main component in the Fusion Reactor.

To stabalize the anomaly and lower it’s gravitational effect a Gravitational Stabilizer can be used.