I would send myself a test e-mail to see what folder it is going. Then test it out by opening it.
Opens often aren't reported if the images aren't loaded by the client- so it isn't always a perfect metric. How many delivered emails were there?
How many emails did you send out? Were they to the same company? While I agree that not even one open is weird, you are right that they may be caught in the SPAM filter. Or if you have no images in the email, there would be no reason do download them and thus download the 1px box to register the open. Or it could be the recipients inbox settings.
If it's all text like Erik said, Opens are tracked by links / Images loading / called actions. Not them clicking to open your email. A good way to prevent this from happening is throw your logo in your email somewhere or even a super small transparent PNG in the footer (or something) but I would recommend your logo :)
If you have images in your email but still aren’t seeing anything. You may just need to wait a little bit. Or contact Marketo Support.
Litmus, Google Analytics are both great for tracking email as well. Just a thought :)