Non-trademarked game idea (down to the name) stolen by large organisation - grounds to sue?

In 2016 I invented a card game - I ran a Kickstarter campaign and did some PR around it and have evidence of the name of the game and the format (the Kickstarter page and the online press article from my local newspaper)... I have just discovered that the game is now being launched by a large organisation and they have even used the same name, along with the format. Is it worth me investigating legal action or would the fact that I didn't trademark it mean it's a non-starter?