Hi Leah,
> Is there a way for me to have a default page when there is not a selection in the map or any of the widgets? Just so that when someone opens the dashboard, there is nothing in the details ...
This is not possible in Operations Dashboard at this time. Please vote for the enhancement and add your comments to request it for the Details element,
https://community.esri.com/ideas/15725
This helps the Dev team prioritize what functionality to add in future releases.
Hope this helps,