By default the application to open ‘.dmg’ files should be ‘DiskImageMounter’ [not ‘Disk Image Mounter’], not ‘Disk Utility’. To return ‘DiskImageMounter’ as the default application to automatically process a ‘.dmg’ file:
If you are getting a terminal windows when you double click on a DMG file then something has switched the Default program from Installer.app and or DiskImageMounter.app to terminal. Right click on the DMG and select Get Info. In the windows that comes up find the Open with section then select DiskImageMounter.app and click Change All. DiskImageMounter will launch when you doubleclick an image to be mounted. There's no reason to search for it in Finder, and anything in CoreServices isn't found by Spotlight (to protect users from trashing anything essential from there mainly).
1- Right click on any ‘dmg’ file -> Get Info’ menu item. A ‘Get Info’ window will appear.
02. From the ‘Open with:’ popup menu (you may have to click on the disclosure triangle to see it) select ‘DiskImageMounter’.
[Or, you may have to click the popup menu’s ‘Other…’ menu item and navigate to the ‘/System/Library/CoreServices/’ folder to select ‘DiskImageMounter’.]
03. Click on the ‘Change All…’ button. An alert box will appear.
04. Click on the ‘Continue’ button.
05. Close the ‘Get Info’ window.
06. Double click on any ‘.dmg’ file to test whether MacOS X actually accepted the changes you just made.