By "it" I mean the onomatopoeia だんだん. I am still a beginner and cannot provide you with the difference between だんだん with と or without as of now. You may try to find similar answers on Stack exchange for using onomatopoeia with と) or you could wait for someone to answer your question. But I am certain that the usage of と is because だんだん is an onomatopoeia
– APKAug 27 '20 at 12:41