When friendship disappears then there is a space left open to that awful loneliness of the outside world which is like the cold space between the planets. It is an air in which men perish utterly.

This quote is talking about how when we lose a friend, the loneliness that follows can often feel like a tremendous emptiness. This emptiness can be compared to the vastness of space, which is devoid of life and has nothing to help fill the void. Loneliness can be overwhelming, leaving us feeling isolated and helpless, as if we are facing the world all alone. This feeling can be so severe that it becomes debilitating.