Sounds like a strange bug, but you can fix this as follows;
Hope this helps!
If the previous suggestions did not solve your problem, try to open your project in the Web Modeler (Beta). As you will see the display there is a bit different.
I don't know if it is possible for an old project to access the Web Modeler without issues. Be aware of that please. The Web Modeler is still in Beta.
When you zoom out in the domain model do you see the these entities?
Another idea in addition to the one Lukas mentioned: search for something about the entity (the name or an attribute name) and see if the entity appears in the search results. If so, you can double click on it in the search results to go to it.
Is this entity somewhere used in a page/microflow action/..?
In that case;
Just wondering; are you sure the entity is in the module you are looking at? ( I know, stupid suggestion, but Stupid suggestions have saved my life before)
None of these suggestions thus far have worked.
Here's a screenshot of the domain model:
Notice, there is nothing on the screen and there are no scrollbars. I've tried zooming in and out, going to the entity or attribute from another page, etc, etc, etc.
Here's a screenshot of the domain model security (clearly showing that there are in fact entities in this module):
If the entities are off the screen, have you tried creating a new entity and dragging it off the screen to extend domain model area? This will trigger the scroll bars on the domain model screen.
Can you submit this to Mendix Support? I'm sure there's a way to fix it with the Mendix Model SDK, but really it shouldn't happen (and I've never seen it before). Make sure you share the project with them.