If you are unfamiliar with the term, omnipresence means God is everywhere. The word omnipresence isn't in the Bible, but the concept is.
Psalm 139 is a really good psalm teaching God's omnipresence. I have learned this fact about God in the past and I'm sure I've acknowledged it as a good thing, but I've recently been comforted by it.
If I belong to Him, it doesn't matter where I am. He is there as well. My son gets scared of the dark and I'll admit that I don't like to be in the basement in the dark and may or may not run up the stairs like something is chasing me if the lights go out. Verse 12 of Psalm 139 says "Even the darkness is not dark to You, and the night is as bright as the day. Darkness and light are alike to You." He is with me in dark places.
It can be actually kind of scary to think of God being everywhere though because I know that he sees my sinfulness. What if my heart is the dark place?
The end of the Psalm is comforting for me too. David, the psalm writer, asks God to search his heart and know his anxious thoughts and look for his hurtful ways. Why would he want God to show him His sinful thoughts? So that he can repent and be led in the everlasting way.
God's presence is to be a comfort because He loves us. He is the light we can cling to in dark places and if that dark place is our heart, He will clear out the dark and replace it with light if we repent and let Him be our God.