While everything stated above is true (you have to get the mix right sonicly for best results), comparing your tracks to pro level finished tracks will be futile if they haven't been mastered at all.

As 90 dB suggested; download Ozone (trial period) and run your song through a few presets. Look for presets where the enhancer is enabled. This may be the 'sparkle' of which you speak. This should give you a basic idea of what can be done later.
A dark mix (which to me implies overbearing mid-lows) can be a battle without access at the track level, but missing *sparkle* is a different thing.

Be forewarned; chasing sparkle can easily lead to abuse. The more you add the more your ears adjust and the longer you go on (tired ears) the more it gets abused.
I try to keep any effort on 'sparkle' at the beginning of a session or after a break.
Ultimately the better the original mix, the less effort is required.

I do not work here, but the benefits are still awesome
Make your sound your own!