system
Given the following post, respond as if you're commenting on Reddit:
human
How can we know that the Cosmic Microwave Background (CMB) is persistent throughout the universe?

* How can we tell that the radiation we detect is not just present in our immediate vicinity?
* Wouldn't we have to make sky survey measurements in interstellar space to be sure?
gpt
Well, one could think that the microwave background is something local, but it would be akin to a scientific conspiracy theory.

We see it coming from every direction in the sky, and it is independent of stars or galaxies. We have a pretty good idea where it originated.

It is possible that super intelligent aliens are smartly irradiating us - and only us - with the microwave background from multiple nearby locations while avoiding detection by other means, but it's not a particularly logical assumption to make. The simplest assumption is that it is the blackbody radiation of the universe from the time when it was very young and hot, and it permeates all space.