Transparent Areas when exporting as PNG
I'm relatively new to Illustrator CC 2018, and I'm having an issue with transparent areas showing up when exporting my file as a PNG. For example, the file below will

export as the following image with extra transparent area at the bottom.

Even when I extend the bottom of the rectangle (the gradient background) to extend past the bottom edge of the artboard, the transparent area will still be there when exporting. This happens to all my files on a seemingly random side of the PNG. Is this intended behavior? The file in this example is 3840x2160px. I've searched this forum and on Google, but I couldn't find a solution.Thank you!
PS: What are the green lines called so I can remove them? ![]()
