"Alien space bats" ("ASBs") is a neologism for plot devices used in alternate history to mean an implausible point of divergence.
"Alien space bats" originally was used as a sarcastic attack on poorly written alternate histories seen as being implausible.
The term eventually evolved into a reference to deus ex machina to create an impossible point of divergence.
[1] Examples include changes to the physical laws of nature, time travel, and advanced aliens interfering in human affairs.
[9][10] In a review of Dies the Fire, Dale Cozort addressed the perceived implausibility of the novel by saying, "Just say to yourself, 'The elder gods or alien space bats took our toys away and that’s all there is to it.