In Slack, open the account preferences by clicking the ^ in the lower left to expand the menu, then the "Preferences" link. Under the "Advanced Options" in the modal that displays, change the "Channel List" list option from "Show all channels" to the middle one, "Hide any channels, DMs, or groups which have no unread activity."
OMG, NOW I can finally see everything that needs to be read. YAY!.
