What evidence is there, apart from the Bible, that the ten plagues of Exodus happened? Very little, as it turns out.
This site quotes two ancient Egyptian texts as supporting the general proposition: The Admonitions of Ipu-Wer‘ (see also this table for other references), and ‘The Prophecy of Nefer-Rohu’. But both these texts are dated to a time before the Exodus events are believed to have happened.
There are also naturalistic explanations for some of the plagues, but not for all of them. And what could have killed all firstborn males (not eldest but firstborn) without a covering of blood from a slain lamb?
But if you are wondering how God could plunge all but one part of Egypt into darkness ‘so thick you can feel it’ for three days, have a look at one of the many online videos from a March 2011 sandstorm in Kuwait: