Overlay div with position absoulte and height 100% not moving with the scroll

please look here and click the on button.

.darkClass
{
    background-color: white;
    filter:alpha(opacity=90); /* IE */
    opacity: 0.9; /* Safari, Opera */
    -moz-opacity:0.9; /* FireFox */
    z-index: 2000;
    height: 100%;
    width: 100%;
    background-repeat:no-repeat;
    background-position:center;
    position: absolute;
    top: 0;
    left: 0;

}

You will see that the overlay div is covering the whole screen as desired, but when I scroll it isn't moving with the scrolling as expected.

I would like to see the grey overlay moving with my scrolling.

what am I missing here CSS-wise?