Ghost kitchens, which are also known as cloud or dark kitchens, allow restaurants to prepare food solely for delivery. This can be critical for businesses seeing strong demand as consumers' appetite for food delivery grows. Third-party food delivery sales grew 138% in December, according to analytics firm Second Measure. Start-ups like Kitchen United or Travis Kalanick's CloudKitchens house multiple restaurant brands within one location and tout their models as more efficient, lowering labor and rent costs for eateries. Meanwhile, virtual restaurant brands are found only on third-party delivery apps, relying on those platforms for marketing rather than a physical storefront. Food from these brands is made either in a traditional restaurant kitchen or in a dark kitchen. For some struggling or failing restaurants, virtual brands have turned out to be lifesaver.