Sometimes the AI empires want to get on your good side as if your decisions are decided by the same opinion modifiers they have, and they will send gifts. It usually comes as a gift of a couple of thousand energy credits. For the player, it's quite easy to milk them for much more by simply not answering the offer. The empire will continue to send the same offer even before the previous one has expired. This way you can wait something like 2 months until the first offer is about to expire, at that point you may have around 7 pending offers which you then accept, to get 20k to 30k credits. I don't know how the AI is paying for it, if it's cheating or actually has all of it stockpiled, but regardless, you will get the money, and it's an exploit that should probably be fixed.