Particle density anomaly
A particle density anomaly was an anomaly that was also identified by the Borg classification 34792. Such an anomaly was encountered in the Delta Quadrant by the  in 2376. 

The anomaly encountered by Voyager that year, was believed to have been formed by the collision of two neutron stars, which created a class 9 neutronic wavefront that extended 3.6 light years and traveled at a velocity of two hundred thousand kilometers per second. 

The effect Voyager felt from the anomaly was an increased exposure to neutron radiation, which not only required the crew to receive inoculations against, but also a disruption to the plasma flow, leaving the ship unable to jump to warp. Since the ship was unable to outrun it at impulse speed, Captain Kathryn Janeway chose to "batten down the hatches" and ride the storm out by creating an inverse warp field and "drop anchor." ()

See also 
 Neutron blast
 Neutronic storm
Astronomical phenomena