...
Announcements are classed as new until the point which the user clicks on the title from their inbox to show the full content of that announcement.
Dashboard tile
The Inbox tile can be added to dashboards to provide a way to display announcements directly on a user’s dashboard.
If users are given the permission to dismiss an announcement then it is this only has an effect within the dashboard tile and not the main inbox. When a user dismisses an announcement it is hidden from the tile and moved to their archive, which is accessed from the bottom of their inbox dashboard tile.
...
All announcements that are contained within their archive will remain there up until the point which they are set to expire, if this date has been set.