Jessica L. Nielson, Sean Carroll, Fredrick Barrett, George Musser, Karl Friston, Rich Oglesby, and Brad Burge offer answers in Hopes and Fears. George Musser:
The holographic principle doesn’t mean the universe isn't real. It just means that the universe around us, existing within spacetime, is CONSTRUCTED out of more fundamental building blocks. “Real” is sometimes taken to mean “fundamental”, but that's a very limited sense of the term. Life isn't fundamental, since living things are made from particles, but that doesn’t make it any less real. It’s a higher-level phenomenon. So is spacetime, if the holographic principle is right. I talk about the holographic principle at length in my book, and I discuss the distinction between fundamental and higher-level phenomena in a recent blog post.
The closest we come in science to “real” or “objective” is intersubjective agreement. If a large number of people agree that something is real, we can assume that it is. In physics, we say that something is an objective feature of nature if all observers will agree on it – in other words, if that thing doesn’t depend on our arbitrary labels or the vagaries of a given vantage point (“frame-independent” or “gauge-invariant”, in the jargon). For instance, I'm not entitled to say that my kitchen has a left side and a right side, since the labels “left” and “right” depend on my vantage point; they are words that describe me more than the kitchen. This kind of reasoning is the heart of Einstein's theory of relativity and the theories it inspired.
Could we all be fooled? Yes, of course. But there's a practical argument for taking intersubjective agreement as the basis of reality. Even if everyone is being fooled, we still need to explain our impressions.