Is it permissible for me to use the wireless internet network of the factory where I work to browse the internet, download files, and upload them without the knowledge of the factory owner, given that this does not incur any additional cost for the factory, and my use of it at night does not affect the factory's usage?

The wealth of a Muslim is not lawful except with his willing consent. The internet service in the factory belongs to the factory owner, and it is not permissible for workers to use it except with his explicit or known permission, even if it does not result in an increase in cost, and whether the usage is at night or during the day. However, if it is known that the factory owner does not object to the use of the network, especially after work hours, then there is no harm in that.