Pads.

Pads.

are Atmospheric notes in this category ?

Layering & Stereo Width (in moderation) is your friend.

Using whitenoise extremely low in the background with a touch of reverb can help create a dirty bed texture that can help the track from feeling completely empty.  I mainly use it during the driving parts in my track. 

The other things as mention in here is reverb and pads especially.  That can help.  Try using FX plugins like Camelspace, Fabfilter Volcano/Timeless 2, and/or CamelPhat 3.  These can help aid in creating long lasting FX elements to sounds you attach them to.  Perhaps also bounce those sounds out and the use the FX's before or after.  Experiment!

idk man, sometimes empty space can be a good thing when you use it correctly. Theres a quote from a famous jazz musician, either miles davis or john coltrane i cant remember, but he basically says his solos aren't about the notes he plays, its about the space in between them. I think this mindset can be applied to mixdowns too

Quote from: Mat_Zo on January 06, 2016, 11:36:40 pm

idk man, sometimes empty space can be a good thing when you use it correctly. Theres a quote from a famous jazz musician, either miles davis or john coltrane i cant remember, but he basically says his solos aren't about the notes he plays, its about the space in between them. I think this mindset can be applied to mixdowns too

Amen. Debussy said that music is the space between the notes!

was about to reply smth similar, emptyness often is a good thing, i love to use those empty spaces over time to make little bits after impact

^ Agree with virtually all responses... and i will add depending on the genre you are making, what you decide to fill the empty space with may change. If i threw a dart at an EDM dartboard and it landed on progressive house, I might fill the space with pads, white noise, reverb tails, delay, etc... Trap maybe different... etc.

Also this might happen to me because I have nothing really powerful in the mids 600 down to 300 or so. This might be the lower octave of a synth chord or a lower voice on a melody. This is if it feels empty in its core guts. If it feels empty in the surround, or the air around it, then take a look at the reverb/delay of any stabs or pads you might have. Those tails can exist if you chain them a bit, even if the chaining isn't audible, it will allow them to exist in the track's mix without causing mud.

*P.S. Also, if you feel it is "missing something", not to be confused with "It feels empty", then it usually (in my personal experience) is the bottom end (sub to low bass OR a bass octave on your leads or stabs (which will be higher). Perhaps the bass and kick were not mixed first, or you have done something to the dynamics of the track that promote the mids and highs and washes out any bass, or you have phasing that is canceling the low end. Or, maybe you moved your room around and forgot to align your monitors and seat in the correct spots, and all that omnidirectional content is being canceled.

The three space related concepts that I am bumping around are:
1. "Feels Empty" - investigate pads, reverb/delay, stabs, octaves.
2. "Something is missing" - Investigate bottom end, consider adding vocals.
3. Emptiness as content: The use of emptiness in your art is a powerful device. But this is not a problem, but a tool.