What is evil?
Why does God allow evil?
These are some of the questions we all ask our selves.
My answer is this:
Being an Electrical / Electronic Tech. I tend to be very technical and pay a lot of attention to details like:
We do not measure a void, we measure the matter that is in it. We use a ruler to measure how long the matter is and a scale to measure how heavy it is. 
A void is simple the absence of any matter.  Go ahead try and measure "nothing".
Did you get that?  A Void is the absence of matter!  God did not Create the Void, it is nothingness, God Created the
Matter that fills the Void!
When we measure Temperature, we measure heat!  A thermometer, as it's name implies (Thermo, is heat) measures heat. We do not measure cold, because we do not have anything that can detect cold. 
Cold is simply the absence of any Heat.  Go ahead try and measure cold, with out a Thermometer or any other heat measuring device.
Did you get that? Cold is the absence of Heat!   God did not create cold, He Created Heat to fill the Cold Void. 
When we measure "Light" we measure foot candles, or "Light", we do not measure "Darkness".
Darkness is simply the absence of "Light"!   Go ahead try and measure "Darkness".  No you can't use lumines that is
just another measure of light.  Darkness does not exist, therefore it can not be measured, just like cold and a void.
Did you get that? Darkness is the absence of Light!   God Created the Light to fill the Darkness.
Do you see where I am going with this?
Death is the absence of Life!  God Created Life to fill the Lifeless Void.
A vacuum is the absence of all gasses / matter. God Created gasses and matter to fill the vaccume of space.
Now am I off the original subject, let's see?
What is Evil?  It ain't nothing it does not exist!!!
Evil is the absence of God!   God is Goodness that fills the Cold, Dark, Lifeless Void that was and still is nothing!
Without God, there is nothing left, but the nothingness we call Evil!  This nothingness we call Evil is like the empty vacuum of Space it is everywhere and nowhere all at the same time.  We can not live in the nothingness or vacuum of Space, just like we can not live with out God!

 No God, Know Evil,
Know God and there is No Evil.
Now we have run God out of our Schools, our Homes, many of our Churches and now we are trying to run Him out of our Nation and we wonder why God allows Evil.  Man is the one that allows Evil, not God!  Where ever He is, Good is, and no Evil can exist, just like where ever there is Light the darkness is pushed aside.
John 8:12
12 When Jesus spoke again to the people, he said, "I am the light of the world. Whoever follows me will never walk in darkness, but will have the light of life." 
(from New International Version)