Blood was silently spreading on the floor in a dim room.
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means extend so the verb vocab version is to extend.
Does the kanji look familiar, kind of like 広い, perhaps? This word shares the same reading as 広い, and the meanings are kind of similar too! That means 拡がる is read as ひろがる.