Allison, do you use the standard Marketo unsubscribe link/page or do you lead people to a custom unsubscribe page? If you aren't going via the standard Marketo route, then you will show zero unsubscribes.
We have a totally separate email preference center, so in all of our email program statuses we track unsubscribes, and then manually enter those people into our reporting. It's definitely a bit tedious, but if you aren't using the standard Marketo route for this then you'll have to do this.
You can set this up in your campaigns by having a smart campaign looking for a (triggered) data value change of your email preferences, and a (filter) clicked link in email - constraint: link is or contains something to do with your unsubscribe URL. The flow step would be to change program status to unsubscribed. It still won't show up in the email send reports, but at least you'll be able to track who unsubscribed and from which emails.