The vastness of the universe (i.e., cosmic vastness) can evoke polarizing existential experiences. Some people report elevating experience, such as awe, elevation, and self-transcendence, whereas others experience existential distress, such as feeling insignificant, powerless, and vulnerable. The purpose of this thesis was to examine three main research questions: 1) How do people react to the vastness of the universe? 2) What is it about witnessing cosmic vastness (i.e., mediators) that elicits polarizing experiences? 3) What is it about a person and their individual traits (i.e., moderators) that causes people to react differently? Two studies were conducted to investigate these questions. Study 1 was a primarily qualitative in-lab study where participants watched one of two videos depicting the vastness of the universe. Participants answered open-ended questions about their experiences, along with some preliminary quantitative questions, and these responses were used to inform what experiences were measured in Study 2. Study 2 was an online quantitative experiment in which a cosmic vastness video was compared to two other videos (Earth nature vastness and neutral control). Study 2 also examined mediators and moderators that explain the relationship between witnessing cosmic vastness and both elevating experience and existential distress. Both studies found that facing cosmic vastness can elicit positive and negative experiences. Study 1 found that most participants reported positive (93%) and negative experiences (68%), including a substantial degree of elevating experience (58%) and existential distress (46%). Study 1 also found several cognitive responses that were good candidates to be mediators in Study 2, including small self, need for accommodation, experience of the unknown, and existential contemplation. Study 2 showed that a cosmic vastness condition elicited greater levels of elevating experience than the neutral control condition and greater levels of existential distress than both the neutral control and Earth nature vastness conditions. Results from both studies also found significant polarization in participants responses, such that half of the participants reported more elevating experience, and half of participants reported more existential distress. Mediation analyses in Study 2 demonstrated that the four cognitive responses each mediated the positive relationship between witnessing cosmic vastness and both elevating experience and existential distress. Furthermore, moderation analyses revealed that self-esteem moderated the relationship between witnessing cosmic vastness and elevating experience, whereas both self-esteem and meaning in life moderated the relationship between cosmic vastness and existential distress. This research provided unique contributions to literature on how people react to vast stimuli that has the capacity to be existentially threatening. Further implications of these results are discussed, as well as how these results may generalize to other areas of research.