Subspace echo
A subspace echo was a natural phenomenon whereby the reflection of a long range communication (via subspace) reached the source some time after the original transmission and, at first glance, appeared to be a unique transmission in and of itself. The phenomenon was aptly named as it was similar to a traditional echo of sound.

The phenomenon was referenced by Crewman Mortimer Harren on Stardate 53764.3 when Captain Janeway and three other crew members on an away mission encountered a dark matter lifeform when trying to establish communication. ()
echo