Are FPGAs for experimentation alone?
I also read an article that they are used for testing purposes alone.

That is so ridiculous that I think you misunderstood the article.  FPGAs are used for various applications, including data processing in specialized applications, and as glue logic in low volume applications where developing a fixed function ASIC would not be viable.  Open up all kinds of electronic equipment and you will find an FPGA, or often many.

Also please tell me the disadvantages of the FPGAs compared to the ASICs.

Compared to an ASIC implementing the same function, an FPGA is slower, uses more power and costs more money per unit. The advantage is that it is much cheaper in low volumes and can be reprogrammed inexpensively.

Are FPGAs slowly being used more in many fields like military, space, image processing etc?

They have already been widely used in those fields for decades, although use in space can be tricky due to radiation hardness.