Sometimes, when you're recording a song, you're looking for an open and spare sound on purpose. But other times, you may want the sound to be more "filled-in" - wider or thicker - if what you want is a thin or empty sound. One excellent way to do this is to add a "pad," which is a sound that is sustained and held out over a long period of time (my definition), such as a string section or organ. Often, it is possible to add a pad to your mix and allow it make an almost unconscious impact on the listener, who will sense a larger or thicker sound without knowing why.
I usually use a virtual instrument to add pads to my mixes. Lately, my plug-in of choice has been Omnisphere, by Spectrasonics, which has hundreds of preset sounds, as well as the ability to alter or create your own sounds. I load an Omnisphere instrument on its own track in Reaper, then play the instrument with an external MIDI keyboard.
Below is a video by Joe Gilder showing you some other ways to thicken up your mix, either with a plug-in/virtual instrument like I usually do, or with a real organ.