To a person with anxiety issues, one of the most unnerving ideas is the idea of infinity. The thought that there is no end and no definable border to space or reality is most unsettling. The pure scope of our existence can be crippling if one is allowed to dwell on the topic.
For example, driving down the interstate you will encounter hundreds of people. Each anonymous face that you pass has their own story. Not just a story about going to the mall today, but maybe a story of thirty years of misfortune or a life filled with luck. That person might be dying. They might be pregnant with their third child. They are not just an extra in a movie or an NPC in a game.
Every day you rub the edges of a vast pool of stories and humanity. You could spend your entire life attempting to understand and catalogue the stories of others without even making a dent in the human population. Beyond each house is a block. Beyond each block is a street. Beyond each street is a city, a county, a state, a country, a continent, a planet, a solar system, a galaxy and then ever onwards into a vast universe that has no comprehendible edge.
Thoughts like this can occupy and overwhelm an idle mind. The sheer vastness of our universe can paralyze the mind the anxious mind.