Stop saying I don't know ! Instead try these 3 alternatives.

If you use I don't know for every unknown answer, you literally kill the conversation and it also sound like you just don't care about it.

Try saying these 3 alternatives to sound more expressive.

1. I'm not entirely sure, but let me find out for you.

It is best for school, colleges or work. It shows that you have no clue about the answer but you can find the answer.

2. I'm actually not familiar with that could you share a bit more? It shows you are curious to learn about the topic and  it encourages them to share more about the topic.

3. Finally if you don't wanna use your head and still sound honest you could say "I'm totally blanking on what that right now."

So, make sure to use them next time in your conversation.