Alien Clue: Visual Void with Intense Heat
Visual telescopes have found a massive region devoid of light. However, this same region emits great amounts of heat.
The reason: An advanced civilization is harnessing all the light power of those stars, most likely through Dyson Spheres, which are huge megastructures surrounding a star collecting all of its light. However, these immense power collecting megastructures radiate intense heat. That is exactly the situation that this void reveals and there are currently no other viable hypotheses to explain it.