Hello Kartik,
position: absolute and then top:50% and left:50% places the top edge at the vertical center of the screen, and the left edge at the horizontal center, then by adding margin-top to the negative of the height of the div, i.e., -100 shifts it above by 100 and similarly for margin-left. This gets the div exactly in the center of the page.
#outPopUp {
position: absolute;
width: 300px;
height: 200px;
z-index: 15;
top: 50%;
left: 50%;
margin: -100px 0 0 -150px;
background: red;
}
<div id="outPopUp"></div>
Hope this work!!
thank You!!