Monday's Riddle! On the Dark Side...

Here's Monday's Riddle! Can you guess the theme this week? Depending on the time of day, the answer could be right next to you ; ) ~ F
Always there, right with you, reflecting back all you do. I am dark, though made of light, and almost always within your sight. I follow along while you run, and when you're quiet, we're both done. There are times when I will go, but not for long, as you well know. Once the sun pops out again, then I'll be your closest friend.
Who am I ?  
I already gave you a clue!  hint, hint : ) FEAR NOT THE UNEXPECTED