Final answer:
Ptolemy's Earth-centered theory remained the popular theory for the structure of the universe for about a millennium until Copernicus developed the heliocentric theory.
Step-by-step explanation:
Ptolemy's Earth-centered theory, also known as the geocentric theory, remained the popular theory for the structure of the universe for about a millennium.
It was not until Nicolaus Copernicus developed the heliocentric theory in the 16th century that Ptolemy's theory began to be challenged.
Copernicus' work, published shortly before his death in 1543, marked the beginning of the Scientific Revolution and led to a shift in our understanding of the universe.