I've also been surching for .Xdefault on my system and found out that there isn't one. Maybe you should search for .Xresources which (I think) is equivalent (sorry if that's not true). But since both files hold information how some programs like xterm, xclock should act/look ... I simply created my own and searched for avaliable resources in /etc/X11/app-defaults, copied and edited.
Hope it helps.