I'm pretty much certain that intelligent life exists somewhere in this universe, but the chance of that life coming to Earth is almost zero.
Literal
This universe [in-には] intelligent life-forms [subject-が] exist [nominalizer-の] [topic-は] nearly-certainly no-mistake [quotative-と] think [but-が], that life-form [subject-が] Earth [to-に] come possibility [topic-は] almost non-existence.
A measured SF-adjacent opinion. まず ('first of all,' 'almost certainly,' 'more or less') is an adverb with several senses; here it's the 'almost certainly' sense, paired with 間違いない ('no doubt'). まず間違いない = 'pretty much certainly,' 'almost beyond doubt.' 殆ど ('nearly, almost, mostly') is the kanji form of ほとんど. 無し ('non-existent,' 'none,' 'without') is the classical noun form of 無い ('not exist') — survives in written and formal contexts. 殆ど無し = 'nearly zero,' 'almost non-existent.' The two halves of the sentence create a concession structure: the speaker believes in alien life but doubts alien contact.