The secret of silk-making was known only in China until about 550 BCE.

Here's some follow up information:

China was very isolated due to their natural barriers, such as the Himalayan mountains, Gobi desert, and a massive plain to the west. They as well have a vast coast line to the east.

This made China very isolated for many years, although it provided protection from invaders.

After the discovery of silk, and after many other countries had found out about it, the Silk Road was made.

The Silk Road is a trade route that carried Silk to other parts of Europe and Asia from China. The Silk Road ended the period of isolation in China, after this China had started to become very populated, like it is today.