You forget just how much space there is in a galaxy and how much stuff is in it: When a wave of energy impacts something; It is reflected Or absorbed (Or if the energy is high enough or composed of small enough particles)----> it is slowed down/slightly dissipated. Planets are INCREDIBLY large and massive somethings that can stop or absorb a very massive amount; Also there is gravity; gravity ALSO pulls in energy into its center of mass. So unless you direct that energy to targets you lose practically ALL of it before 10% of your targets are even hit. Also unless energy in question is SELF AWARE once it exits slip space; there IS NO GOING BACK INTO SLIPSPACE for that specific portion of energy. If you were to TRY and blanket an entire galaxy with ANY form of energy for EVERY SINGLE 2 millimeter space within that galaxies area; IT REQUIRES MORE ENERGY+MASS [b][i][u]THAN THAT GALAXY EVEN CONTAINS[/u][/i][/b] ^-----cuz guess what; all the energy+ matter a galaxy has doesnt even fill up every single LIGHTYEAR within that galaxies area LET ALONE every 2 MILLIMETERS

Perhaps the forerunners have self-aware energy. Wouldn't be the craziest thing they have.