Normally I wouldn't do this but you all are being way, way too kind. I am not taking anything away from the girl or her beauty but you guys are never going to learn anything if someone doesn't point out the error of your logic. The image is too dark because the person posting it didn't care enough to go into Photoshop and adjut the exposure. It had nothing to do with material across the lens or infront of the light source. All material infront of the lens is going to do is soften the image it is your job as a photographer to correct for this either at the time of taking the shot by opening up 1/2 to a full stop dependant of the material or fixing the exposure in an image editing program afterwards.
The image below shows that all you had to do was adjust the levels and you would have been a lot closer to what I am sure you saw in your mind.
Keep shooting guys and sorry if I seam a little harsh but you will never learn if people keep being overly kind.