Is X server running? xdpyinfo: unable to open display "" on Ubuntu 18.04 server

I'm attempting to start a program that requires rendering a window (Unity Game) on a remote server that doesn't have a display attached to it. I wouldn't want to use ssh -X to forward rendering commands to my local machine. I'm trying to run the program in headless mode but still making use of the remote server's GPU, or use Xvfb, but I cannot get it to work. In the first step of this process, I try to **determine whether X server is already running**. Upon sshing into the remote server, I ran echo $DISPLAY and it returns empty string. I thought that this means the X server is not running ([reference](https://stackoverflow.com/questions/637005/how-to-check-if-x-server-is-running)) However, I encounter contradicting information: * When running ps ax | grep X, I get

2149 tty1     Sl+    0:03 /usr/lib/xorg/Xorg vt1 -displayfd 3 -auth /run/user/121/gdm/Xauthority -background none -noreset -keeptty -verbose 3

This means indeed X server is running (because of the Xorg executable). Running ps -C Xorg also shows PID 2149 ([reference](https://unix.stackexchange.com/questions/122067/make-sure-x-server-isnt-already-running)) . * Also, when I run Xvfb :0.0, I get

$ Xvfb :0.0
    _XSERVTransSocketUNIXCreateListener: 
    ...SocketCreateListener() failed
    _XSERVTransMakeAllCOTSServerListeners: server already 
    running
    (EE) 
    Fatal server error:
    (EE) Cannot establis

This again seems to indicate that an X server is already running on :0.0. However (again), when I run xdpyinfo, the utility for displaying information about an X server, I get

$ xdpyinfo
xdpyinfo:  unable to open display "".

and

$ xdpyinfo -display :0.0
No protocol specified
xdpyinfo:  unable to open display ":0.0".

This seems to tell me again that the X server is not running. What is going on? Is the X server running? As you may be able to tell I'm a newbie to X-server related issues. Please feel free point out any problem with my steps above, and suggest ways to resolve this. Thanks! There's a lot to learn.