From the very beginning, Terra's leadership never intended for the population to boom.
Just as the Duke of Solin was aware of the existence of the Star Prison Tianyu, the leadership of the Terra Confederation had long been vaguely aware of the Star Prison Tianyu's existence—at most they were unclear about the prison's exact form and strength, but they understood very well that if a 'foreigner' truly existed who could lock them within the starfield.
Then humans, if they wish to continue existing, should avoid acting rashly.
Until absolutely necessary, until all of Terra's planets have been fully developed, no one should seek to explore the essence of the prison.
