Display on monitor is not fully used. What to do?

You may have to purchase a display adapter so to use another port in the computer.

Check the display to see if other signal-in options exist that may use another cable.

A newer model computer should be able to use a modern display with Thunderbolt

connections; some cables with adapter-ends for use with older displays may work

if the display can be used that way. Not all work even if an adapter may fit the shape.

• Apple Mini DisplayPort adapters: Frequently asked questions (FAQ) - Apple Support

While I have two Mac Mini computers, one is a Late 2005 and the other Late 2012;

both can use an adapter to HDMI so a 19-inch Samsung works for these at full res.

Mini-display port to HDMI should work; if not using Thunderbolt.

One of my displays uses one of these adapters:

Apple Mini DisplayPort to DVI adapter

Some models of display require drivers that would be obscure and not included with

OS X; I have another Samsung that rotates, it is supported but have to use a third-

party control panel app in both older OS X 10.5.8 and OS X 10.9.5. The in-OS X

controls for Brightness and resolution are limited without it.

{Given the newer Mac Mini looks better with a 20-inch Aluminum Cinema, that's what

it has; the older Mini does OK with an 19-inch (more square) Samsung, and it uses

a control software to adjust items the OS X cannot. Newest can use Thunderbolt.}

Not sure if your display is older of those, which may need a separate control software.

There was a firmware update for Mini Display Port to VGA Adapter (itself) :

About the Mini DisplayPort to VGA adapter firmware update - Apple Support

If you do not see all the possible resolutions of the display in your Displays preference

it may be due to the lack of a preferred display adapter. Or an older display limits.

• How to Show All Possible Resolutions for a Display in Mac OS X:

http://osxdaily.com/2015/08/27/show-all-display-resolutions-external-screen-mac/

• About Mini DisplayPort to HDMI adapters - Apple Support

You may have to research the display maker support site for technical specs if you do

not have a printed specification page with line-in info stated. Several different adapters

are available; you could even use a digital TV (within limits) as a display.

• Learn more about troubleshooting display related issues.
• Learn which adapters are compatible with specific Apple computers.
• Learn more about Thunderbolt ports and displays.

The adapter and display technologies need to more closely match those of the Mini.

An older tech display should be supported within limits. Drivers and connections.

PS: See if the firmware update for the adapter you have, will allow it to work.

If the adapter is an Apple product, it would be able to accept the firmware.

Good luck! 🙂

edited 3x

Although I'm not certain how well it may work with your present display, and OS X in the Mini

you could trial use test this old standby to see if it can help better view & use the display...

The trial use period is stated in the maker's site; the shareware cost is 14 Euro afterward.

• SwitchResX - Versatile Tool for Controlling Screen Resolutions: (shareware)

http://www.madrau.com/index.html

• Information about SwitchResX on El Capitan 10.11: (note this requires an extra effort to use)

http://www.madrau.com/support/support/srx_1011.html

{This idea may be harder to implement under El Capitan due to the System Integrity Protection.

A command-line is required, to disable S.I.P. -- Note details in the second link above, to use this.}

Other general info & tips - not necessarily related to this issue - may be found here:

• Help/How-to Guides for Mac/OSX/iOS Users - everything from AirPlay to Thunderbolt info:

http://www.xlr8yourmac.com/tips/MacBasics_Guides.html

Previous versions of Mac OS X found custom options in display settings with SwitchResX.

It may or may not be worth acquiring a license for use beyond trial run; but it may surprise.

In any event...
Good luck & happy computing! 🙂

Stop using the VGA input on your monitor. VGA is not a digital output, it is an analog output. To fill the screen, it would require you to make manual adjustments using the monitor's resizing adjustments.