Police found the 20-year-old's body on Saturday afternoon after other students in the International House complained of a smell. The last time he used his keycard to enter the dorm was on February 7.
"I don't understand how the people that live next door, and the people who were responsible for his well-being could have not seen that he has been gone for a week," said International House resident Jordan Ginsburg.
"It's uncomfortable," said resident Ryo Atnei. "I can't believe no one noticed."
Barnes is from Pittsburgh. An autopsy performed Sunday was inconclusive, according to the Cook County medical examiner's office, and toxicology reports are pending. His death is not believed to be a suicide, according to the medical examiner.
There is no evidence of foul play, according to the University of Chicago, which emailed students about the death. Karen Warren Coleman, vice president for campus life and student services, wrote that the cause of Barnes' death was under investigation.
"University staff are making every effort to understand the circumstances surrounding Nicholas' death," Coleman's email said.
Barnes was majoring in Germanic studies and had studied in Austria in the fall of 2012, undergraduate Dean of Students Susan Art wrote to students, staff and faculty in another email. She noted his involvement in the campus literary publication called Sliced Bread Magazine.
"Nick will be painfully missed," Art wrote. "He was an excellent student, admired by faculty and peers alike."
The university planned to host a campus memorial and Barnes' family members were planning a funeral in Pennsylvania, university officials said.
The University of Chicago, located on the city's South Side near Lake Michigan, is a private college with more than 5,600 undergraduate students and nearly 10,000 graduate students.
