Pros/Cons of using multiple databases vs using a single database

I was working on a new project which has the requirement to use 7 databases, arguing that performance, stability, optimization are more easily implemented. While I don't agree, I'm having trouble collecting good arguments to use a single database (splitting the tables into logical domains). One argument I have so far is data integrity (I can't use foreign keys between databases). What are good pros/cons for using a single or multiple databases? **[summary so far]** Arguments against multiple databases: - Losing data integrity (can't use foreign keys over databases) - Losing restore integrity - Gaining complexity (db users/roles) - Small odds server/database will go down Solutions: - Use schemas to separate domains. - POC: Use dummy data to prove the point in 7/1 db's execution plans