The phrase "dark smile" can evoke a sense of foreboding or mystery. To capture this feeling, there are a variety of alternative expressions that could be used. For example, a "sinister grin" suggests a malicious intent behind the smile. A "smirk of malice" or a "grin of evil" convey a similar sense of malevolence. Alternatively, a "shadowy smirk" or a "murky smile" suggest a hidden agenda or unreadable emotions. For a more supernatural feel, a "demonic grin" or a "vampiric smile" could be used. Overall, there are many synonyms to choose from to convey a sense of darkness behind a smile.