空虚 (kōng xū): hollow; emptiness; meaningless

The pace of life in the big city is fast, but he often feels an inner emptiness, as if something important is missing.

After retirement, she suddenly felt that her days were empty, and she didn't know what to do every day.

He was addicted to online games, trying to fill the emptiness in reality with a virtual sense of achievement.

In the dead of night, an inexplicable feeling of emptiness welled up in his heart, making it hard for him to fall asleep.